13. Progress in SSRL Operations
by Piero Pianetta
Appendix B Self-Evaluation FY2006

FY2006 User Experimental Run

In the FY2006 user run, the facility proved to be exceptionally reliable, providing very stable beam for a very high fraction (96.2%) of the scheduled time. This is an exceptional achievement for a new storage ring. The user run commenced on November 28, 2005 and continued through August 7, 2006, and the SPEAR3 storage ring operated at 3 GeV/100 mA and provided 60+ hour life times. (The average uptime over the past five years was 96%.) During the FY2006 run, scientists on 345 different proposals received beam time in a total of 1,002 experimental starts involving approximately 1,700 users, with approximately 900 users on-site or remote accessing beam line equipment. Approximately 66% of the users came from universities and other laboratories in the United States, 15% from DOE and US government laboratories, 5% from US industry, and 14% from international institutions.

Distribution of Proposals Receiving Beam:
Materials Science 14%
Physics 5%
Chemistry 16%
Polymers 2%
Medical Applications 5%
Biology/Life Sciences 45%
Earth Sciences 3%
Environmental Sciences 5%
Optics 1%
Engineering 3%
Other 1%

SPEAR3 Accelerator Improvements

The accelerator improvements for SPEAR3 are focused in four areas: 1) characterizing and improving accelerator performance and reliability at 100 mA to maximize beam quality for users; 2) developing and implementing a new lattice optics configuration that accommodates a magnetic chicane having two small vertical beam size waists (the double-waist chicane optics, or DWC optics) in the east long straight section; 3) characterizing and optimizing accelerator operation at 500 mA; and 4) preparing for the implementation of top-off injection with beam line stoppers open.

Injector – Work continued in FY2006 to improve the reliability and operation of the injector at 3 GeV. The injection timing system was modified to equalize booster ramping efficiency for all bucket timings. Work continued to characterize and stabilize gun and linac operation. A feedback system was implemented to stabilize the booster White Circuit. Booster performance was assessed and plans for its realignment were made. A study of the stability of all injector power supply systems was initiated. Additional improvements include rebuilding the BTS injection line (to remove several vacuum windows), upgrading transport line beam position and intensity monitors, enhancing the stability and monitoring of powered devices, completing a pulsed signal monitoring system, implementing an EPICS control system access to injector component control and readback variables, and installing a second klystron for the gun/linac system. The design of simpler and more robust booster kicker pulsers has begun and plans for developing a laser-heated-cathode rf gun are in progress. The Accelerator Physics group is studying booster beam capture efficiency and has proposed a minor lattice modification that could improve performance. Studies of other booster improvements, including the powering of unused sextupole magnets and the possible addition of a subharmonic rf cavity, are under way. The injector improvement project will continue in FY2007 and beyond.

Turn-Turn Beam Position Monitors (BPMs) – Commissioning and development of the user software interface for the turn-turn BPM system is continuing in FY2006. The test tone calibration system will be commissioned and the true performance of the processors is being quantified. The turn-turn BPMs will then be integrated into the orbit feedback system.

Synchrotron Light Monitor (SLM) – The SLM beam line received first light in January 2006. The optical bench components were commissioned and the beam line component alignment was refined. The first measurement of beam bunch length was performed during the user run and low alpha experiments were performed in which the bunch length was shorted to 6.5 ps with 100 mA stored current in SPEAR. The upgrade and realignment of the BL2 pinhole camera system was completed in early spring of 2006.

Beam Scrapers – The new horizontal and vertical beam scrapers were commissioned and used to characterize beam lifetime, injection apertures, and minimum apertures for future insertion devices. The scrapers will continue to be a valuable diagnostic for beam size, lifetime and aperture studies.

LION Development – Commissioning of the Long Ion Chamber (LION) system continued during the FY2006 user run. Tests of the system with high injected beam current were carried out in early spring 2006. The LION system must be connected to the Beam Containment System before the injected beam current limit can be raised to enable faster 500-mA fill times.

High-Current SPEAR3 Tests – SSRL received authorization from the DOE Site Office to conduct SPEAR3 operation with currents up to 500 mA, above the official safety envelope value of 100 mA allowing routine running of SPEAR3 with beam lines closed for accelerator studies. A successful test of the Double Waist Chicane lattice (see below) at 500 mA was conducted in February 2006 with no observed problems.

Orbit Control – The first phase of the fast orbit feedback system, using only the 54 Bergoz averaged-orbit BPMs, was completed. A test of the system demonstrated successful feedback operation with a bandwidth approaching 100 Hz. Further work was done to optimize the SVD-derived inverse response matrix and digital filters as well as to maximize the frequency response of the orbit corrector power supplies. Work continued in order to provide an operator interface anddata acquisition applications for the new fast orbit feedback system.

The orbit monitor processing system has a temperature dependence that has been demonstrated to cause orbit instability when the feedback system is active. A project to build temperature-controlled rooms and install finer temperature regulation in the processor equipment racks was completed in time for testing before the end of the run. Initial tests showed that the cooling system reduced the processor temperature variation from several degrees Celsius to less than a degree, thus decreasing the processor temperature-related orbit instability. Further tests during the 2007 run will quantify the degree of improvement.

Hydrostatic level sensors (HLS) were installed a few strategic areas in the SPEAR3 tunnel to monitor the vertical motion of tunnel floor and some beam line components. If this system proves to be useful for detecting component motion, more sensors will be added in the future and their signals will be used in the orbit feedback system.

Double Waist Chicane (DWC) Lattice – The DWC optics, temporarily installed without chicane magnets during the FY2005 shutdown, was tested successfully for the first time in December 2005. While the lifetime was a few percent less than that for the normal lattice, this was anticipated in the optics studies and is acceptable. This lattice was characterized and optimized so that it will be used with the new BL12 undulator and beam line which was installed during the 2006 summer shutdown. It has already been demonstrated to work without problems at 500 mA and with the “dispersion leak” optics variation that reduces the emittance from 18 nm-rad to 12 nm-rad. The design and fabrication of vacuum chamber and chicane magnets needed for the full DWC implementation are being completed during the 2006 shutdown. Most of the components have been installed, together with the BL12 in-vacuum undulator. The installation of the beam line that utilize the downstream chicane straight section is underway.

Top-Off Injection – The design of the system needed for top-off injection with beam line stoppers open was begun in FY2006. This injection mode, with minimal interruption to users, will enable more frequent beam injection to limit beam current variation, minimizing the variation of thermal power on beam line optical components and improving beam stability for users. Work has begun to improve the injector systems to accomplish this injection mode (see above). An extensive study of beam loss modes was initiated to determine what radiation safety components will be required to inject beam into SPEAR3 with open beam line radiation stoppers. Significant changes to the Personnel Protection System, including the Septum Interlock, will be needed to implement the top-off system. This work will enable the first phase of top-off injection - a mode that will maintain beam current constancy in SPEAR3 to a few percent. A second phase of top-off development that would enable maintaining SPEAR3 beam current constancy to less than 1% will most likely require large-scale improvements to the electron gun and possibly to the booster. These changes will not commence until FY2007.

SPEAR3 Performance and Lattice Upgrades – The accelerator physics and engineering groupscontinued to study and tune the SPEAR3 accelerator to maximize its performance and stability. This work includes investigating the sources of beam instability (power supplies, rf systemcomponent vibration and temperature-related motion, etc) and ongoing efforts to characterize beam dynamical behavior as a function of lattice and insertion device parameters. With regards to the latter, a detailed study of the deleterious effects of the new BL13 EPU on beam properties, and possible cures, is in progress in collaboration with physicists at other light sources. The results of this study will be incorporated into the design of the BL13 EPU system.

An investigatory study of the possibility of circulating short bunch electron bunches (<~1 ps) in the SPEAR3 storage ring was initiated. A new isochronous lattice configuration was developed that would preserve the short bunch length, and provide the option of circulating the bunches for some small number of revolutions (<1000), as opposed to storing the beam, will be analyzed.

Gun Test Facility – The GTF continued to support the LCLS injector group through FY2006. Experiments performed included novel in situ methods to improve the cathode performance, experiments to eliminate the correlated energy spread produced by the gun, and beam-based screen resolution measurements. New diagnostics will continue to be developed including the electro-optic bunch length measurement as well as testing new diagnostics such as digital cameras and bunch charge monitors. The laser will continue to be used to test transverse pulse shaping techniques, the LCLS streak camera and methods to improve the laser pointing stability.

Additional ultrafast electron diffraction experiments were also planned. Improved detectors were tested and the pulse length reduced to improve the temporal resolution. The useful operating range of the gun for electron diffraction has also been explored.

Beam Line and Facilities Improvements

BL1, 2, 3, 8 (bend beam lines) – Upgrade activities on bend magnet beam lines 1, 2, and 8 were limited to those essential to keep the beam lines operating with higher SPEAR3 current. In particular, during the 2006 shutdown, the BL2 beam position monitor was upgraded for improved stability and the BL8 beam position monitor shielding was upgraded for 500 mA operations. BL3 will remain closed.

BL4 – The BL4 upgrade continued with the fabrication of components and is scheduled for installation during FY2007. The BL4 upgrade is partially funded by DOE BER.

BL5 – The BL5-1 M₃ refocusing mirror system has been installed and commissioned as has the shielding for 500 mA operation. No other significant upgrades are scheduled.

BL7 – The 500-mA upgrade installation was completed with the temporary BL7-2 LN monochromator and the beam line was commissioned. Assembly of the BL7-2 sagittal focusing monochromator will be completed and the monochromator will be installed and commissioned at the beginning of the FY2007 run. BL7 and BL 7-3 upgrades are largely funded by NIH NCRR.

BL10 – While no significant upgrades are planned, some cooling enhancements of zero order beam masks downstream of the BL10-1 monochromator have been scheduled.

New beam lines under development:

BL12 – The in-vacuum ID was delivered and installed in the ring as were the remaining dipole magnets and associated vacuum chamber required to produce the SPEAR3 orbit chicane. The hutches are being erected. The remaining optical components will installed during October 2006. It is anticipated that the beam line will start commissioning by November 2006. The computing infrastructure required to support this beam line is being installed. A large-area CCD detector was procured with an October 2006 delivery date. This beam line is funded by the California Institute of Technology through a gift from the Gordon and Betty Moore Foundation.

BL13 – The ID fabrication will continue. The beam line front end design will be completed and fabrication will commence. The M₀ mirror system will be designed and the optic ordered. Design of the in-alcove beam transport system will start. Rather than order a new monochromator for BL13, it has been decided to relocate the BL5-1/5-2 spherical grating monochromator, associated slits, and refocusing optics to BL13. This relocation is planned for the summer 2007 shutdown. During the remainder of FY2006, the BL5-1/5-2 gratings and grating cooling system will be analyzed for applicability to BL13. If new gratings are required, the gratings will be ordered in FY2006.

SSRL Instrumentation and Control Software

XAS Instrument Control System Software and Computing Developments – The new ICS software has been installed on BL7-3. This system will run on the Microsoft Windows operation system and will form the basis for the upgrade of other beam lines as the new standard SSRL beam line control system. The system will be based on VXI instrumentation, controlled using a National Instruments USB2 interface.

It is anticipated that the new ICS software running on the Microsoft Windows XP operating system controlling CAMAC hardware using the existing Grand Interconnect hardware will be installed on a least one experimental station.

Development of the SSRL dedicated beam line network will continue. The initial design, consisting of several VLANs has been implemented initially on BL7. Experience gained during the operation of the new beam will fine tune the design which will be extended to all beam lines in the coming years.

Legacy OpenVMS based programs will still be available, but will be run from a server style computer, displaying on the workstation at the beam line. Projects will be initiated to either port or replace such applications to an operating system independent model.

Computers and Networking – The SSRL network has been extended to Building 130. Deployment of SAN storage technology is ongoing. Further planning for the fiber-optic network backbone upgrade to 10 Gbit/s data rate will be performed and the availability and cost will be analyzed. The beam line network cabling and infrastructure system will be reorganized to meet cable plant improvement requirements. Some central windows servers will be upgraded and a Windows 2003 Server will be deployed. Central services will be available on Itanium-based servers. In collaboration with the SLAC networking group, a possible upgrade of the wireless network infrastructure will be planned. New web-based SSRL user administration applications will be developed and deployed to the public.

Facilities and Infrastructure – Following the recommendations of the final safety review for the LN distribution system, an oxygen deficiency monitor network is being added and integrated into the Building 120 fire alarm upgrade project. Implementation of insulated piping to the LN-cooled monochromators will then begin. The funding for the SLAC SLI SORIP infrastructure project has been received The air handler supplying cool air for the SPEAR3 power supply building (Building 118) does not have the cooling capacity necessary to support SPEAR3’s planned growth. It will be replaced with a higher capacity unit and additional ductwork will be installed.

Facility Research and Development

Inelastic Scattering and Advanced Spectroscopy Facility for SPEAR3 – An Inelastic X-ray Scattering and Advanced Spectroscopy Facility is being developed that will eventually be located at a new SPEAR3 insertion device beam line. Various techniques complementary to the current spectroscopy programs at SSRL will be carried out at this facility. They include X-ray Raman scattering (XRS), resonant inelastic X-ray scattering (RIXS), selective X-ray absorption (S-XAS) and X-ray emission spectroscopy (XES). XRS will widen the range of absorption spectroscopy on low Z samples, traditionally performed in the soft X-ray range, to systems and sample conditions where the penetration of a hard X-ray probe is essential. XRS can thus provide unique new insight for, e.g., studies of carbonaceous systems related to fossil fuels and hydrogen storage under in situ conditions, water and aqueous systems in ambient and extreme conditions, high pressure phases of gases and the formation of methane hydrates. RIXS spectroscopy is a novel technique to study in detail the local electronic structure and spin states of, e.g., 3d transition metal compounds with hard X-rays. As compared to conventional K-edge spectroscopy, it can better isolate lowest unoccupied molecular orbital (LUMO) resonances and has less lifetime broadening along the energy transfer axis. Furthermore it provides L-edge/M-edge like information. S-XAS, such as site-selective EXAFS, combines the chemical sensitivity of XES with EXAFS to provide more detailed structural information in mixed valence systems. S-XAS can also be used to extend the k-range of EXAFS beyond an absorption edge that otherwise would limit the data collection, hence yielding more accurate determination of neighbor distances. XES contains chemical and structural information complementary to XANES. All of these techniques are valuable in the study of a wide range of systems including man-made and biological catalysts as well as correlated systems.

Internal DOE-BES funding was allocated for instrumentation development in FY2005. Additional funding was obtained through non-DOE grant awards. First, as part of the SSRL Structural Molecular Biology program proposal to NIH-NCRR and DOE-BER (described in the KP11 FTP) funding was awarded to: a) make the unit compatible to perform emission scans as required for XES and RIXS and b) purchase analyzer crystals for the various proposed applications related to biological 3d transition metal systems. Second, in collaboration with Prof. Anders Nilsson (PI), an NSF proposal (NSF CHE-0518637 1096374-2-QANAB) focusing on research on water in ambient and extreme conditions was submitted and funded. Funds were used to upgrade the XRS spectrometer with parts for a second multicrystal component which will double the efficiency to a total of 14 analyzers.

Three Si(553) analyzers were purchased for work on Cu Kβ XES and the upgrade of thespectrometer for XES in addition to XRS capabilities was undertaken. This required the purchase and integration of two vertical stages for simultaneous scanning of goniometer and detector in order to obtain X-ray emission spectra. In addition, a stand-alone alignment unit based on a Newport table was built on which the complete XES setup is placed. This unit can be attached tothe BL6-2 hutch table, allowing for a fast turn around with other users at BL6-2. SSRL thus has developed and implemented XES (resonant and nonresonant) as well as XRS capabilities at BL6-2. In addition to the XRS work, XES on Mn and Cu Kβ lines can currently be performed. The potential of the technique was demonstrated with the observation of a clear spectral shift for compounds with H2O versus OH^- ligands. Commissioning work on the new XES spectrometer was also performed at APS where, in addition, experiments on Zn and Mn proteins were carried out. Multiple beam-time proposals for SSRL-based XRS and XES work were submitted in November. Finally, the process of submitting a science-based proposal to the DOE for an undulator-based facility was initiated. Several potential users from groups are actively engaged in contributing to the scientific case for this proposal. Plans for development of dispersive optics for pump-probe type XRS, XES and RIXS experiments will be initiated as will implementation of in situ and high pressure instrumentation for XRS studies. The development of the science-based funding proposal to DOE for an undulator-based facility will be completed and the proposal will be submitted by the end of FY2006.

Molecular Environmental and Interface Science – Molecular Environmental and Interface Science (MEIS) research at SSRL focuses on the fundamental interfacial, molecular- and nanoscale processes that control contaminant and nutrient cycling in the biosphere with the goals of elucidating local and global elemental cycles and anthropogenic influences on the environment. Knowledge of these processes is required to develop contaminant remediation technologies and environment-friendly industrial processes. Mass and energy flow through surfaces and reactant transformations, often driven by solar inputs, occurring on nanoparticles, and mediated by bacteria, are major research themes in this field, and offer discovery opportunities for novel remediation technologies and energy capture, conversion, and storage materials/processes. Key areas of investigation at SSRL include: (a) structural chemistry of water and dissolved solutes, (b) structural chemistry and reactivity of environmental nanomaterials (biominerals, oxide and sulfide minerals, biofilms, and organic materials), (c) reactions at environmental interfaces, including sorption, precipitation and dissolution processes that affect the bioavailability of heavy metals andother contaminants, and (d) microbial transformations of metals and anions. SSRL-based MEIS research utilizes synchrotron-based X-ray absorption spectroscopy (XAS), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), X-ray standing wave (XSW) spectroscopy, and photoemission spectroscopy (PES). These techniques provide unique capabilities to probe structure/composition/function relationships in complex environmental systems.

The Brown Group has continued studies at SSRL in the following areas (1) abiotic and biotic oxidation pathways of pyrite (FeS2) and cinnabar (HgS) surfaces; (2) formation of ternary surface complexes of dicarboxylic acids and metal ions on metal oxide surfaces; (3) studies of the reactivity of nanoparticles of hematite and cinnabar to heavy metal contaminant ions; (3) interactions of metal ions with biofilm- and organic polymer-coated metal oxide surfaces under in situ conditions; (4) XAFS spectroscopy studies of heavy metal contaminated soils and mine wastes, including mercury speciation in mine wastes from the California Coast Range, zinc and arsenic speciation in soils from the Carnoules region of southern France, and uranium speciation in soils and sediments from Chihuahua, Mexico; and (5) XAFS, micro-XAFS, and micro-XRD studies of uranium in the Hanford Vadose Zone.

In the area of environmental nanomaterials (Bargar), research focused on synthesizing and characterizing bacteriogenic Mn oxides having different sizes and properties. To support a SSRL-based post doc for this research, a 5-year interdisciplinary proposal was submitted to the NSFCRC program (Chemistry Division), in collaboration with three other institutions (UC Berkeley, Princeton, and Oregon Health and Sciences University). Research will be initiated to study structure/reactivity relationships of bacteriogenic nanoparticulate UO2 (post doc supported byDOE-BER EMSP) as part of a collaborative multi-disciplinary investigation of the fundamental chemical factors controlling the long-term release of uranium at remediated field sites. A two-day user training workshop entitled, “Synchrotron X-ray Scattering Techniques in Materials and Environmental Sciences: Theory and Application” was held in May 2006.

The instrument development goals achieved in FY2006 were to integrate the X-Y-Z scanning stage and detectors for rapid XRF imaging measurements. User commissioning for the μ-XAS and μ-XRF systems will be initiated in winter/spring 2006 and completed during June/July 2006. The procurement for an area X-ray detector for μ-XRD measurements has been initiated.

Strongly Correlated Materials – The program of angle-resolved photoemission spectroscopy (ARPES) study of strongly correlated electronic materials continues to be very active and productive in its two main tasks: scientific research and advanced instrumentation development and operation in support of the research. The goal is get critical information about the strongly correlated materials that cannot be obtained by any other means.

During this period, the instrumentation development takes two forms: i) Develop and design a new beam line that will bring the ARPES capability to a new level. A scientific case has been made for a new beam line. ii) Upgrade current experimental end-station to have better sample temperature control, and photoelectron detection efficiency. Instrumentation development has been very important to the success of the program, and significant progress has been made on both fronts during the period. We will have a new experimental chamber, an improved low temperature sample manipulator, and an improved spectrometer/detector. These improvements will also lower the cost of the proposed future upgrades of the beam line. These instrumentation innovation efforts have benefited not only the research program described below, but also other user programs at SSRL and ALS.

Research wise, the primarily focus is the many-body interactions that are important to the mechanism for high-temperature superconductivity; however, we have also extended the work to other correlated oxides.

Work has been performed to systematically investigate the evolution of Fermi surface and quasiparticle dynamics through a topological transition in correlated oxide Sr_2-xLa_xRuO₄. In collaboration with a group at University of St. Andrews, we have performed a joint study which combines de Haas-van Alphen (dHvA) and ARPES to track the carrier doping evolution of the correlated electron system as the material is doped through a critical point in the band structure. We investigated the relationship of proximity to this critical point and an evolution from Fermi liquid to non Fermi liquid behavior. The significantly improved momentum resolution enables this investigation.

We uncovered evidence for small lattice polaron formation by looking at the Frank-Condon type of broadening in Ca₂CuO₂Cl₂ and Sr₂CuO₂Cl₂. Small lattice polaron formation and its interplay with magnetic interactions in undoped and underdoped materials have been theoretically suggested for a long time, direct spectroscopic evidence for this behavior does not exist yet. By performing a temperature dependent investigation, we expect to make progress on this subject.

Important progress was made to understand the electronic structure of a novel multilayer cuprate material where the CuO₂ layers are self-doped. These materials exhibit a number of surprises as they are very high temperature superconductors although simple valence counting would have put them in the insulating regime. We have uncovered a novel form of self-doping and a number of surprises associated with it.

Important progress was made in understanding the role of B1g phonon coupling and superconducting transition temperatures in multilayer materials such as Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂Ca₂Cu₃O₁₀. We found that this mode coupling is much stronger in materials with multilayers of CuO2 planes in their unit cells and with higher T_c, while this coupling is much weaker in cuprates with single CuO₂ layer and lower T_c.

Important progress was made in uncovering a phantom Fermi surface and its nesting instability in Ca₃Ru₂O₇. The delicate interplay between various degrees of freedom and their intricate roles in the rich physical phenomena is at the heart of physics in complex oxides. In Ca₃Ru₂O₇ , high resolution ARPES data reveal well defined quasiparticle bands of unusually low weight, emerging in line with the metallic phase of the material below ~30K. At the structural phase transition temperature of 48K, we find clear evidence for an electronic instability, gapping large parts of the underlying Fermi surface that appears to be nested. Metallic pockets are found to survive in the small, non-nested sections, constituting a low-temperature Fermi surface with two orders of magnitude smaller volume than in all other metallic ruthenates.

Important progress was made in understanding Fermi surface and quasiparticle excitations of Sr₂RhO₄. We find well-defined quasiparticle excitations with a highly anisotropic dispersion, suggesting a quasi-two-dimensional Fermi liquid like ground state. A quantitative analysis of the ARPES derived band structure is in excellent agreement with dHvA and specific heat data. This work presents one of the rare quantitative comparison (the other being Sr₂RhO₄) between ARPES, transport and thermodynamic data down to few percent level.

We have observed doping dependent coupling of electrons to bosonic modes in the single-layer Bi-cuprate Bi₂Sr₂CuO₆. We compare for the first time the self-energies in an optimally doped and strongly overdoped, non-superconducting single-layer Bi-cuprate, Bi₂Sr₂CuO₆. Besides a strong overall weakening we also find that weight of the self-energy in the overdoped system shifts to higher energies. We have presented evidence that this might well be related to the coupling of c-axis phonons which are un-screened at optimal doping, being particularly sensitive to the rapid change of the c-axis screening in the doping range.

Chemical Physics of Surfaces and Liquids – The main focus of this research program is to use X-ray and electron spectroscopies to address important questions regarding chemical bonding on surfaces and in aqueous solutions. Photoelectron spectroscopy (PES), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS) and X-ray Raman spectroscopy (XRS) provide an atom specific projection of the electronic structure. Problems related to systems in catalysis, energy technologies, electrochemistry and molecular environmental science are studied using XES, XAS, XRS and density functional theory (DFT) calculations. Probing hydrogen bonding and the structure of liquid water in aqueous systems are new and novel applications of X-ray spectroscopic techniques. Instrument development is an important part of the activity to provide new spectrometers, and enable measurements at high gas pressures and at liquid interfaces.

Detailed studies of water adsorbed on TiO₂ and Fe₂O₃ at ambient pressures using the differential pumped XPS systems continued, as have hydrogenation studies of carbon nanotubes for investigations of the potential of carbon based materials as hydrogen storage materials. Studies ofwater at high pressures and temperatures, different pH and in various aqueous solutions have provided new information to address structure, hydrogen bonding and electronic structure of water in the bulk and in the influence of ions.

Molecular Adsorbates on Surfaces. XPS and XAS studies were performed on water adsorbed on Cu surfaces at ambient conditions. At pressures of 1 Torr, water dissociates to form a co-adsorbed oxygen-OH-water overlayer on Cu(110). The relative concentration of the different species depends strongly on temperature and relative humidity. On Cu(111) the interaction of water with the surface at 1 Torr and room temperature is extremely weak, resulting in a bare surface with no adsorbed water. These results show that in order for water to wet a Cu surface, it is necessary to form OH groups on the surface allowing for strong hydrogen bonding between water and OH.

Catalysis. Using a combination of density functional theory calculations and XES and XAS for nitrogen on Cu and Ni surfaces, a detailed picture has been obtained of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces ascatalysts for chemical reactions can be related to certain characteristics of the surface electronic structure, in particular the center of the d band is important.

Water in Aqueous Systems. New models of the structure of liquid water based on XAS and XRS experiments which go against the existing understanding based on theoretical simulations were proposed. New experiments with XRS have demonstrated important isotope effects between H2O and D2O which indicate that the discovered asymmetry in hydrogen bonding in water could be related to quantum effects in the motion of the hydrogen atoms. New experiments of water using photoelectron spectroscopy have been performed using both valence and core levels providing insights into the electronic structure rearrangements due to hydrogen bonding.

Instrument Development. A new UHV surface science end-station was completed and installed at BL5. A highly efficient soft X-ray spectrometer optimized for C, N and O K-edges has been designed and is under construction and a differentially-pumped high-pressure cell using cryogenic technology that can be inserted into this UHV system is being assembled.

Development of Resonant Coherent X-ray Scattering – The scientific motivation for the development program outlined below is the investigation of the critical fluctuations of a magneticdomain structure at the magnetic-paramagnetic phase transition. This research falls into the general area of critical fluctuations at phase transitions for which theory predicts that the order parameter diverges at the transition temperature. For magnetic phase transitions this implies that the domain size should diverge, i.e., right at the transition temperature a single magnetic domain should extend –momentarily– over the entire sample. However, this has never been observed experimentally. One of the major reasons for this lack of experimental proof is that impurities and defects limit/influence these fluctuations. This limitation can be overcome by studying critical fluctuations in ultra-thin films with quasi 2-dimensional magnetization. Such ferromagnetic films can be prepared essentially defect free by epitaxial growth with thicknesses of only a few monolayers. A further advantage of studying a thin film is that the fluctuations are expected to be much slower in thin films than in bulk materials. To investigate the nature of these critical fluctuations, the following four experiments will be undertaken:

Resonant Small Angle Scattering of Incoherent Soft X-rays
Resonant scattering at the dichroic L3 absorption edge of magnetic transition metals will yield statistical information about the magnetic domain structure such as average domain size and domain shape. Hence, when investigating the temperature dependence of this statistical information close to the transition temperature, the average domain size of the critical magnetic fluctuations can be derived.
X-ray Photon Correlation Spectroscopy (alias Dynamic ‘Light’ Scattering)
By scattering of coherent photons, information about the dynamics of the fluctuations can be derived from the time dependence of the scattering intensities. A third generation synchrotron light source like SPEAR3 will enable time resolving dynamics occurring on the microsecond time regime.
Ultra-Fast, High-Resolution, Lensless Imaging of Magnetic Domain Structures
One potential application of SLAC’s upcoming X-ray free electron laser LCLS will be ultrafast, high-resolution lensless imaging. This will allow recording of femtosecond snap shots of the magnetic domain structure at and in the vicinity of the magnetic phase transition. A series of such images will enable distinguishing “real” magnetic fluctuations from defect-pinned fluctuations.
Ultra-fast X-ray Photon Correlation Spectroscopy
At LCLS, a beam splitter and a delay line will be used to obtain two femtosecond short X-ray pulses separated by a variable delay ranging from a femto- to a few picoseconds. Using both these pulses for lensless imaging of the magnetic domain structure will reveal the dynamics of the magnetization fluctuations on a femto- to picosecond time scale.

Commissioning of the rebuilt BL5-2 continued, which also included commissioning of the dedicated end station for coherent soft X-ray scattering. Double pinhole test scattering patterns were recorded to characterize the coherence properties of the beam line. In addition, a novel implementation for coherence measurements based on a non-redundant array of pinhole structures was developed and successfully applied.

The feasibility of phase contrast imaging in resonant soft X-ray holography was demonstrated by imaging of a magnetic domain structure. The important implication of this achievement is that sample damage can be reduced significantly by using phase instead of amplitude scattering contrast.

In addition, multiple reference beam Fourier transform holography has been developed. Since an image is obtained simultaneously from each reference beam, the effectiveness of the imaging technique increases linearly with the number of reference holes. This allows to further reduce the required dose for imaging on the nano-scale, which is in particular important for radiation sensitive samples like organic materials.

The thin film preparation chamber assembled during FY2005 has been commissioned. First ultrathin magnetic films for the investigation of critical magnetic fluctuations have been grown. In preparation for these experiments, temperature control, earth field compensation, and an optical MOKE system to characterize the magnetic properties of the thin films were developed.

Small and Wide Angle Scattering Studies of Soft Matter & Colloids – A proposal for a new SAXS/WAXS Materials Science beam line at SSRL has been developed in response to the burgeoning demand for the technique in a host of modern nano-scale science applications. The beam line’s design takes advantage of the accrued knowledge of the decades of experimental scattering from synchrotron sources in the field of materials science. Detailed plans concerning its physical specifications and configuration are already in place: with a double crystal monochromator (interchangeable between silicon [111] crystals and multilayers) and a horizontal and vertical focusing toroidal mirror and five meter path length downstream of the sample environment. This will provide a beam ~10¹³ photons able to probe correlation lengths within physical media up to half a micron in size at time resolutions of 100 ms or less, thereby opening up a facility for a whole new range of science. Some of this new science that would be made accessible by this geometry would include many pore size distributions (spatially) and matrix complexations (temporally) that are currently beyond the abilities of modern facilities.

Nanoparticles for Environmental Sorption Control. This project focused on the nano-scalestructural chemistry and environmental chemical dynamics of bacteriogenic manganese oxide (MnO) nanoparticles. Investigations at BL1-4 addressed the relationships between particle size and MnO structure and stability and the factors controlling the colloid chemistry of MnO nanoparticles in aquatic systems. Insofar as Mn plays unique and important roles in local and global elemental cycles, including those of C, S, N, Fe and numerous other trace metals, the knowledge gained from this project will contribute towards our understanding of the dynamics that control the chemistry of our soils, natural waters, and atmosphere. Initial measurements of bacteriogenic Mn oxide particle sizes in bacteria/mineral mixtures were made allowing the feasibility and test techniques for dispersing particles to be assessed.

Reflection Geometry SAXS. The capabilities of the new experimental hutch permitted collection ofthe first SAXS data in a reflection geometry, studying controlled growth of silicon nanotubes intended for nano-scale thermoelectric applications. In an effort to access the pores and achieve significant alignment, a method was developed for producing vertical pores out of the plane of a substrate. Cubic mesoporous titania was believed to have produced an ultra-flat hexagonal pattern from its 111 face that could be used to surface nucleate vertical growth in the silica 2D hexagonal system SBA-15 and hence the creation of accessible, aligned pores. Early data, in the reflection geometry, permitted characterization of the packing arrangement and aspect ratio of these nanotubes, which revealed difficulties in the synthesis, leading to misaligned tubular structures, which remain to be corrected.

Nanoporous Metallic Media. Nanoporous metals were fabricated electrochemically by selective dissolution or ‘dealloying’. When immersed in a suitably aggressive reagent (e.g. nitric acid), themore active metal was removed leaving behind a ‘sponge-like’ bicontinous network of the noble element. We have studied AgAu and CuPt alloys, which formed nanoporous Au or Pt, respectively. SAXS has been used to characterize the pore sizes and the pore morphology as a function of dealloying time (1 minute to 3 days). We found that with increasing dealloying time the average ligament spacing increases and the morphology develops into a bicontinous network which then coarsens. In a related study, diffraction has been used to characterize the strain that develops during dealloying.

Structural Properties of Novel Materials – Several major subgroups of materials are explored in this area, including the local structure of non-crystalline materials, thin films, and nanoporous materials.

Non-crystalline Materials – A second experimental run was performed on the structure of liquid water. Several experimental improvements were made, including using an energy of 19.6 keV instead of 12 keV, which increases the maximum momentum transfer to 19.7 Å^-1. Secondly, asmaller, 600 micron orifice was used in the water jet. The smaller water diameter results in better energy resolution within the diffracted beam analyzer, which allows for better resolution between the elastic and Compton components of the diffracted beam. We also implemented a thermal bath for the recirculated water beam enabling us to measure the scattering both at room temperature (23 C) and at 5 C, very close to the density maximum of 4 C.

The analysis of the scattering from water is underway. Some of the challenges needing to be met during the analysis include the correct separation of the elastic and Compton components of the scattered beam. The advantage of increased energy resolution improves but does not eliminate the overlap between the elastic and Compton components. In a small region of momentum transfer(roughly between 2 and 6 Å^-1) there is overlap of the two peaks. Analytical techniques are being developed to consistently separate the two components. A second challenge is the correct form of the independent scattering. A molecular form factor has been calculated for water vapor, but it is expected that there will be subtle changes in the shape of the form factor for liquid water. Weintend to use the excess electron density at small atomic separation to modify the form factor for liquid water. We plan to submit the work for publication during this period as well as write the doctoral dissertation.

Thin Film Structure – Studies on organic semiconductors have been extended to sexithiophene and substituted oligothiophenes, such as p-tolyl-trithiophene-p-tolyl, and studies will be initiated of other semiconducting conjugated polymers, such as poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’bithiophene], PQT-12, which has a high field-effect mobility.

Nanoporous Metals – Nanoporous metals can be fabricated electrochemically by selective dissolution, or ‘dealloying’. When immersed in a suitably aggressive reagent, the more active metal is removed leaving behind a ‘sponge-like’ bicontinous network of the noble element. Alloys of interest include AgAu, CuAu, CuPt, which form nanoporous Au or Pt for example. X-ray scattering methods (SAXS and GIXS) have been used to carefully characterize the pores and the near-surface structure in these materials.

Charge Density Wave Materials – Rare earth (R) tellurides (RTe₂ and RTe₃) are ideal materials in which to study charge density waves (CDW), since the CDW gap is large and large crystals can be grown. High-resolution X-ray diffraction has been used to study the nature of the incommensurate and commensurate CDWs in RTe₂ and RTe₃ with initially R=La and Ce. From the diffraction data, the atomic displacements, which permit a deeper understanding of the observed thermodynamic and transport properties of these materials, have been obtained.

Ultra-trace and Microanalysis – A micro-focus X-ray mirror system using Kirkpatrick-Baez (KB) optics was commissioned in the back hutch of BL6-2 and first real data studying interplanetary dust particles were collected. Initially, this required a profound understanding of the alignment of the newly upgraded upstream beam line optics illuminating the virtual source of the KB system. This adjustable virtual source slit allows mapping to be carried out at variable focal spot sizes between approximately 2 and 15 microns. Photon fluxes fall in the 10⁸ – 10¹⁰ photon/second range depending on the X-ray spot size, and the accessible energy range covers readily the elements Mg through Br. An aluminum-coated BN window allows simultaneous imaging of the sample with a high resolution optical microscope during the X-ray measurements. A helium gas shower is mounted above the sample to minimize radiation damage (due to ozone generation) as well as spectrum “contamination” by argon fluorescence from air. This shower creates a gentle flow of He without introducing vibrations to the sample mount. The X-ray hutch itself has been temperature-stabilized minimizing possible X-ray beam and/or sample drift during measurement. In addition, the data collection software SUPER was upgraded allowing the collection of the entire emitted fluorescence spectrum for each pixel rather than collecting only predefined single channel analyzer windows for selected elements of interest. This is crucial in particular for studying samples containing a variety of elements such as interplanetary dust particles. In those cases overlapping fluorescence lines in particular from neighboring elements can easily lead to false images of the elemental distribution. Efforts were made to evaluate different software packages for quantitative analysis of the individual fluorescence spectra. A software package developed at the ESRF called PyMca is currently being employed for quantitative analysis. The KB-based experiments in FY2005 were very successful, given the short amount of beam time available for characterization of the optical setup. In addition to optics characterization, elemental maps and trace element spectra were obtained for several thin sections of micrometeorites. These samples included round-robin samples provided by the Bulk Chemistry subteam leader of the Stardust Preliminary Examination Team. Our microprobe has been used in the preliminary examination of the samples returned by the Stardust NASA mission which swept through the tail of a comet (comet Wild 2) resulting in the first successful sample return mission since the Apollo program.

In FY2006, the KB optic-based microfocus facility has opened up for external user groups. A first user group from Canada has already been on line in January 2006, studying Fe distribution in fruit fly brains as a model for human Alzheimer disease. In addition, SSRL scientists performed preliminary test measurements for upcoming user groups studying Cu distribution in mice liver as a model for the Wilson disease, as well as of transition metal distribution in arthropod tools, such as spider fangs or worm teeth.

In addition to the work on the microprobe, a new TXRF chamber was designed, built and commissioned on BL6-2 mostly for studying small samples from the Genesis mission. Despite its hard landing, this mission returned fragmented samples of solar wind collected at the Lagrangian point for over two years. TXRF, due to the ability to distinguish between surface contamination (from Utah desert) and the true implanted solar wind, is an especially effective technique for studying these types of samples. In particular, the sample holder of this new setup was tailored for mounting small (cm²) size samples without adding any contamination. In close collaboration with the PI of the Genesis mission, flight-spare samples and flown Sapphire samples have been analyzed with high detection sensitivity. These indicate that while some of the returned sample pieces have significant surface roughness and contamination, effectively reducing the sensitivity of the measurement, others are sufficiently clean and smooth to enable us to measure bulk solar wind of dominant species (e.g. Fe). Further examination of samples will determine whether minority species will be able to be quantified.

Finally, the proposal to NIH (1 R01 EB004321-01) with the title “A multi-keV X-ray microscopy facility for bio-imaging” for development of a zone plate-based, multi-keV Transmission X-ray Microscope (TXM) facility was funded in May 2005 by the Bioengineering Research Partnerships program of NIBIB. The total award was distributed over four years and started in May 2005. The purchase order for the basic TXM instrument from Xradia Inc. was placed in September 2005. This instrument will provide important and currently at SSRL unavailable capabilities for the study of biological systems in-situ, such as the imaging of elemental distributions within single cells or tissues with high spatial resolution. The full field microscope will operate using photon energies between 5 – 13 keV and exploits the advantages of hard X-rays for 2D and 3D absorption and phase contrast microscopy such as large penetration depth, a large depth of focus, analytical sensitivity and compatibility with wet specimens. Together this will allow high resolution in-situ imaging, tomography and spectromicroscopy without extensive sample preparation. The TXM will be delivered from Xradia Inc. in October of 2006. We are currently building a new X-ray hutch on BL6-2 for this high-resolution microscope. It is anticipated that commissioning will begin in fall of 2006 and by the end of 2006 resolution test structures will be imaged in 2D and 3D with a spatial resolution below 60 nm in absorption contrast.

XAS Studies as a Probe of Electronic Structure / Contribution to Function – X-ray absorption spectroscopy (XAS), at the ligand and metal K- and metal L-edges, is used to determine the electronic and geometric structure of metal-based centers in inorganic and bioinorganic systems. The goal is to understand the involvement of the metal center in catalytic cycles important in industrial and biological catalysis. The experimental approach is complemented by Density Functional Theory (DFT) calculations, photoemission spectroscopy measurements, valence bond simulations and the development of data analysis tools, as well as by other non-synchrotron-based methodologies including MCD, resonance Raman and EPR spectroscopies, where applicable. The combined approach enables significant insight into geometric and electronic structure and their contributions to reactivity. Currently, studies are focused on systems containing copper, iron, titanium, manganese, molybdenum, and tungsten, with the anticipation of extending these studies to other transition metal sites.

Copper –The geometric and electronic structures of two mononuclear CuO₂ complexes [Cu(O₂){HB(3-Ad-5-ⁱPrpz)₃}] (1) and [Cu(O₂)(β-diketiminate)] (2) have been evaluated using CuK- and L-edge XAS studies in combination with valence bond configuration interaction (VBCI) simulations and spin unrestricted broken symmetry density functional theory (DFT) calculations. These systems are related to oxygen activation at a single copper center, as in dopamine βmonooxygenase and peptidylglycine α-hdroxylating monoxygenase. Cu K- and L-edge XAS data indicated the Cu(II) and Cu(III) nature of 1 and 2, respectively. Total integrated intensity under the L-edges showed that the ψ^*LUMO in 1 and 2 consists of 20% and 28% Cu character, respectively, which is indicative of a very covalent ground state in both complexes, although more so in 1. This is consistent with VBCI simulations, which also indicated that the ground state in 2 has more d⁸n-character. DFT calculations showed that the ψ^*LUMO in both complexes is dominated by O₂ character, although the O₂^n- character is higher in 1. It was also shown that the strong donor ligand in 2 is responsible for tuning the molecule towards more Cu(III)-peroxide like character in contrast to trispyrazolyl borate in 1 which leads to a Cu(II)-superoxide species.

Heme-Copper Oxidases – The geometric and electronic structure of the untethered heme-peroxocopper model complex [(F8TPP)Fe^III-(O₂^2-)-Cu^II(TMPA)](ClO₄₎ (1) has been investigated usingCu and Fe K-edge EXAFS spectroscopy and DFT calculations in order to describe its geometricand electronic structure. The Fe and Cu K-edge EXAFS data indicated a Cu···Fe distance of ~3.72 Å. Spin unrestricted DFT calculations for the ST = 2 spin state were performed on [(P)Fe^III-(O₂^2-)-Cu^II(TMPA)]⁺ as a model of 1. The peroxo unit is bound end-on to the copper, and side-on to the high spin iron, for an overall µ-η¹:η² coordination mode. The Fe^III-peroxide η²-bond has two components that arise from the donor interactions of the peroxide π*_σ and π *v orbitals with the Fe dxz and dxy orbitals, which give rise to σ and δ bonds, respectively, while for the Cu site the primary bonding interaction is between the peroxide π*_σ and Cu d_z² orbitals. The π*_σ interaction of O₂^2-with both Cu (η¹) and Fe (η²) provides an effective superexchange pathway for strong antiferromagnetic coupling between the metal centers.

Non-Heme and Heme Iron

Cyanides – Distinct spectral features at the Fe L-edge of the two compounds K₃[Fe(CN)₆] and K₄[Fe(CN)₆] have been identified and characterized as arising from contributions of the ligand π* orbitals due to metal-to-ligand back-bonding Analysis of the L-edge spectral shape, total intensity and energy shift have been used to quantify the contributions of σ-donation and π-back-donation to metal cyanide bonding. The methodology developed demonstrates the application of Fe L-edge XAS as a direct probe of metal-to-ligand back-bonding.

Heme vs. Non-Heme Fe – Fe porphyrin compounds, or hemes, form the basis for electron transfer in a number of biological systems, with the most well-known being the cytochromes, which effect electron transfer by shuffling between low spin Fe(II) and Fe(III). The delocalization of the Fe d-orbitals into the porphyrin ring has been difficult to study spectroscopically because of the dominant porphyrin π →π* transitions, which obscure the metal based d-d bands. Recently, we have developed a novel methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e. differences in delocalization of the different d orbitals) using a valence bond configuration interaction (VBCI) model. Applied to heme systems, this methodology allows experimental study of the delocalization of the Fe d-orbitals into the porphyrin ring. This methodology has been applied to study two model systems [Fe(tpp)(ImH)₂]Cl and [Fe(tpp)(ImH)₂] (low spin Fe(III) and Fe(II), respectively) and have compared their multiplet structure to those of the two low spin non-heme compounds [Fe(tacn)₂]Cl₂ and [Fe(tacn)₂]C_l3. The Fe L-edge spectra are very sensitive to the effects of π donation and π back-donation. The e¹ hole (in heme, D_4h; t_2g, in O_h) in the ground-state electronconfiguration of low spin Fe(III), creates a dominant spectroscopic feature which was quantified in terms of π donation. The L-edge spectrum of [Fe(tpp)(ImH)2] exhibits additional transitions caused by back-bonding, which was also quantified. It was found that heme acts as a substantial �-donor to Fe(III) but only a minimal �-acceptor to Fe(II), indicating the electron transfer involves a hole-based super-exchange mechanism.

Siderophores – In order to overcome the immense difference between environmentally and nutritionally available Fe, many microorganisms produce low-molecular weight iron-chelators called siderophores. Our recently developed L-edge methodology, described above, provides a way of studying the bonding in these compounds. Fe L-edge data on a small set of compounds, K₃[Fe(oxalate)₃], [Fe(pha)₃] and K₃[Fe(catecholate)₃] have been obtained. Results show that both the Fe K- and the L-edges shift to lower energy across the series: K₃[Fe(ox)₃]< [Fe(pha)₃]<K₃[Fe(cat)₃]. This shift indicates a decrease in effective nuclear charge and an increase in electron donation by the ligands. The total intensity of the Fe L-edge transitions decreases across the series, implying that the Fe-O bonds of K₃[Fe(cat)₃] are the most covalent of the compounds. By simulating the shape of the spectra, it has been possible to calculate the σ and π contributions to bonding in the compounds, providing insight into the factors which affect the differences in their thermodynamic stability.

Iron-Sulfur – Ligand K-edge XAS has been used to obtain a quantitative description of Fe-S bonding in Fe-S model complexes and protein active sites. The results of these studies have been correlated to the extent of H-bonding and differences in redox potentials. To develop a quantitative description of hydrogen bonding in Fe-S systems, a series of P450 model complexes, where the amount of hydrogen bonding was systematically varied, were examined by S K-edge XAS. The data show a dramatic decrease in pre-edge intensity with increasing H-bonding to the ligated thiolate. DFT calculations reproduced these effects and showed that the observed changes are in fact solely due to H-bonding to the thiolate ligand. The energy of the H-bonding interaction was estimated to be -2.5 kcal/mol in the gas phase. The rather small H-bonding energy appears to contrast the large change in ligand-metal bond covalency (30%) observed in the data. A bond decomposition analysis of the total energy was developed to correlate the pre-edge intensity change to the change in Fe-S bonding interaction on H-bonding. This analysis showed that the Fe-S interaction energy is greatly reduced due to H-bonding. This effect is greater for the reduced than the oxidized state, leading to an ~350 mV increase in the redox potential. It was found from a VBCI model that E^o should vary linearly with the covalency of the Fe-S bond in the oxidized state, which can be determined directly from S K-edge XAS. The above study was extended to a hydrogen bonded [Fe4S4]²⁺ cube, which had an elongated core structure in contrast to the compressed core structures of most [Fe₄S₄]²⁺ cubes. A decrease in pre-edge intensity was observed for the H-bonded cube. DFT calculations indicated that the change in Fe-S covalency observed experimentally had almost equal contributions from the cluster elongation and the H-bonding interaction. These calculations also indicated that the elongation of the [Fe₄S₄]²⁺cube changes the spin topology of the ground state due to redistribution of the ligand superexchange interactions in the cluster.

The geometric and electronic structure of the active site of the nonheme iron enzyme nitrile hydratase (NHase) has been studied using S K-edge XAS and DFT calculations. Using thiolate (RS^-), sulfenate (RSO^-) and sulfinate (RSO₂^-) ligated model complexes to provide benchmark spectral parameters, the results showed that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO^-species changes upon protonation as the SO bond is elongated (by ~0.1 Å). These signature features were used to identify the three cysteine residues coordinated to the low-spin Fe^III in the active site of NHase as CysS^-, CysSOH andCysSO2^-in both the NO-bound inactive form and the in the photolyzed active form. These results were correlated to geometry optimized DFT calculations. The pre-edge region of the XAS spectrum is sensitive to the Z_eff of the Fe and revealed that the Fe in the [FeNO]⁶ NHase specieshas an effective nuclear charge very close to that of its photolyzed Fe^III counterpart. DFT calculations revealed that this results from the strong π back-bonding in to the π anti-bondingorbital of NO, which shifts charge from the formally t₂⁶ low-spin ferrous center.

Titanium Cyclopentadienyl Catalysts – Ti-TEMPO complexes (TEMPO = 2,2,6,6tetramethylypiperidine-N-oxyl) provide a means for generating Ti(III) complexes by homolysis of the Ti-O bond. The rate of Ti-O bond homolysis depends on the ancillary ligation to the titanium, and it has been determined that bis-Cp-Ti-TEMPO (Cp=cyclopentadienyl) complexes readily undergo homolytic cleavage, while the mono-Cp-Ti-TEMPO complexes do not. Recently, Ti K- and Cl K-edge XAS studies have been applied to a series of Ti-TEMPO complexes (TiCl₃TEMPO, TiCl₂CpTEMPO, TiClCp₂TEMPO). The XAS results indicate that these complexes are best described as Ti(IV)-TEMPO anions. The Cl K-edges show that the replacement of Cl by Cp weakens the remaining ligands, demonstrating a spectator ligand effect which is one factor that contributes to Ti-TEMPO bond homolysis. However, correlation of the XAS results to DFT calculations shows that stabilization of the three-coordinate product by Cp makes a more significant contribution to the energetics of Ti-O(TEMPO) bond homolysis.

PES Studies of Electronic Structure Contribution to Function – The shake-up satellite structure present in core and valence photoemission spectroscopy (PES) data is being used in combination with a valence bond configuration interaction (VBCI) model to experimentally quantify electronic relaxation (i.e. the change in electronic structure of metal complexes upon oxidation) and its contributions to reduction potentials and kinetics of electron transfer. Variable-energy PES (VEPES) experiments provide the tool to maximize the metal contribution, while minimizing that of the ligand, to the valence band region through cross section effects (delayed maximum and Cooper minimum) and resonance enhancement. VEPES data on a series of model iron complexes – high spin [FeCl₆]^4-/3- and low spin [Fe(CN)₆]^4-/3-, [Fe(tacn)₂]^2+/3+, [FeTpp(HIm)₂]^1+/0, [FeTpp(Py)₂]^1+/0– were measured and analysis initiated.

Materials Research

Research carried out by SSRL faculty and staff and associated Stanford faculty and students covers a broad set of disciplines: (1) Complex Materials; (2) Magnetic Materials; (3) Scientific and Educational Gateway Program (through FY2006); (4) Novel Materials and Model Systems for the Study of Correlated Phenomena; (5) Nano-scaled Magnetism in the Vortex State of High-Tc Cuprates; (6) Nano-scale Electronic Self-Organization in Complex Oxides; (7) Nano-Magnetism; (8) Behavior of Charges, Excitons and Plasmons at Organic/Inorganic Interfaces; and (9) Development and Mechanistic Characterization of Alloy Fuel Cell Catalysts. Areas (4) through (8) are collaborative efforts of the SSRL X-ray Laboratory for Advanced Materials and the Stanford University Geballe Laboratory for Advanced Materials.

1. Complex Materials

Z.-X. Shen, S. Doniach, M. Greven, R.B. Laughlin, X.J. Zhou, K. Tanaka, H. Li, L. Lu, K.Downum, Y. Cho, J. Hancock, D. Santiago, D. Schroeter, L. Zhou, X. Zhou, B.A. Bernevig

The team has conducted comprehensive experiments using photoemission (Shen), scattering (Greven and Shen), and theoretical investigation (Laughlin and Doniach) on complex materials, and has made substantial progress in this period. There also are synergetic interactions with the nano-science programs of Greven, especially in the area of single crystal growth, and Shen’s core program on strongly correlated materials.

A major focus of the program is the electron-doped materials that have presented an important challenge to the systematic understanding of high-temperature superconductors. The Greven group has continued its successful effort on the electron-doped superconductor Nd₂-xCe_xCuO₄ (NCCO), built on its early success in growing high-quality single crystals. Recent results include the first determination of spin-correlations in superconducting samples, which constitutes significant progress toward a full understanding of the normal state of the electron-doped superconductors. Since superconductivity in high-T_c cuprates appears in close proximity to the antiferromagnetic phase, it is essential to understand the nature of nearby magnetic ground states. Through careful X-ray and neutron diffraction work, Greven discovered that the oxygen reduction process, required to render NCCO superconducting, transforms a fraction of the crystals into cubic (Nd,Ce)₂O₃, and that the field-effects observed by others and ascribed to a quantum phase transition of NCCO are not intrinsic, but due to this secondary phase. Consequently, the question of genuine magnetic field effects in NCCO has remained a very interesting, unresolved research topic. By studying the magnetic field effect on the spin excitations, Greven was recently able to obtain new data consistent with the absence of field-induced magnetic order [Motoyama et al., preprint]. Moreover, these measurements constitute the first neutron scattering study of the effects of a magnetic field on the superconducting magnetic gap in NCCO. The discovery of spurious magnetism in NCCO is a good example of the benefits of a synergistic growth and scattering effort like that by Greven.

Greven continued the novel use of inelastic X-ray scattering to investigate the collective charge excitations in the model high-temperature superconductor Hg1201, the single-layer material with the highest value of T_c, and of the parent compound La₂CuO₄. This latter work, carried out at the APS and made possible by newly available large Hg1201 crystals grown by Greven, led to the discovery of a remarkably rich structure of electron-hole pair excitations in the cuprate superconductors.

Greven's work on the structural phase diagram and charge-order phenomena in the layered manganite was extended to cover a wider range of doping as well as neutron scattering. These results will allow a comprehensive understanding of the structural and magnetic phase diagram.

The angle-resolved photoemission spectroscopy (ARPES) component of the program (Shen) has two primary tasks, research, and operation of the beam line 10.0.1 end-station at the ALS in support of this research. That activity also benefited a broader community performing ARPES experiments using the end-station.

The focus during this period of time is to understand the nature of collective modes coupled to cuprate superconductors. Data with very high signal-to-noise level, together with a numerical method developed in collaboration with Ward Plummer’s group (Oak Ridge National Laboratory), enabled a glimpse of the phonons and their coupling to electrons in cuprates. This represents the first experimental evidence for multi-boson coupling in cuprates.

Another area of major progress during the period is the new insight on the polaronic behavior of single hole in undoped cuprate. Based on ARPES data from La₂CuO₄ and phonon spectra fromneutron scattering, theoretical calculation by Gunnarsson’s group (Max Plank Institute) show that the spectra are consistent with the material being in the polaronic regime, which is consistent with our earlier finding for copper oxide chloride.

Shen’s group also has made important technical progress in applying synchrotron radiation for high resolution spectroscopy experiments. The resolution and flux density at the ALS have been a critical factor for the program in generating a significant database. On the other hand, this also raises the technical problem of space charging. Shen’s group has systematically characterized this problem and our finding is generally useful in guiding high resolution photoemission experiments in third generation synchrotron radiation facilities.

A major advance by Shen’s group during this period is the discovery of nodal quasiparticle and nodal antinodal anisotropy in layered colossal magnetoresistive manganites. This finding providesdeep new insights on the “pseudogap” phenomena in cuprates. The pseudogap behavior has for a long time been considered to exist only in superconducting materials, and thus is directly related to high-temperature superconductivity. Finding this same behavior in ferromagnetic state suggests that the relationship between “pseudogap” and superconductivity is more subtle. In particular, this finding supports the notion that there are two “pseudogaps”, and the larger pseudogap is in fact a reflection of a competing state to superconductivity. Thus, both superconducting and pseudogap phases are manifestation of the same underlying physics that give rise to the rich and intricate phenomena seen in these complex oxides.

The work of the Laughlin group in 2005 was conducted mainly by David I. Santiago and Zaira I. Nazario. The Laughlin group has studied the phase diagram in cuprates high Tc superconductors.Recent experimental measurements suggest the coexistence of various phases of matter in different regions of the pseudogap regime. Guided by such experiments the group has studied the properties of a spin-density-wave antiferromagnetic mean-field ground state with d-wave superconducting (DSC) correlations. This work concentrates in the case when antiferromagnetic order is turned on weakly on top of the superconductivity, which corresponds to the onset of antiferromagnetism at a critical doping. In such a case a small gap proportional to the weak antiferromagnetic gap opens up for nodal quasiparticles, and the quasiparticle peak would be discernible.

The program looked broadly at the many-body problem in condensed matter system, beyond the high-Tc superconductors and transition metal oxides. Work on the superfluid to Mott insulator transition in bosonic systems has been done, where the phase diagram of a single component Bose Einstein Condensate (BEC) in an optical lattice at zero temperature was obtained. In that work, the discontinuous nature of the transition between the superfluid and the Mott insulator (under certain conditions) was elucidated, as well as its independence on commensuration of the number of bosons with the lattice. Recently, measurements which could be interpreted as such a transition have been performed by Mark Kasevich's group of Stanford University. While, being superfluids, BECs share many properties with superfluid helium, they have never been seen to share the existence of roton excitations which are present in helium. Superfluid helium is close to becoming a solid. The roton minimum is a consequence of enhanced density fluctuations at the reciprocal lattice vector of the stillborn solid. Thus rotons have not been observed in BECs in atomic traps since they are not near a solid phase, but if they are tuned near a transition to a Mott insulating phase, a roton minimum will develop at a reciprocal lattice vector of the lattice. Equivalently, a peak in the structure factor will appear at such a wave vector. The smallness of the roton gap or the largeness of the structure factor peak are experimental signatures of the proximity to the Mott transition.

Laughlin’s group has also done some work on metal-insulator transitions. Specifically, the work was motivated by V₂O₃ and f-electron systems which have phase diagrams in which a line of first order metal-insulator transition ends at a critical point above which the two phases are indistinguishable. Bob Laughlin's Gossamer technique was extended to show that the Gossamer metal in a single band model will describe a metallic phase that becomes arbitrarily hard to differentiate from an insulator as one turns the Coulomb correlations up. Thus one can go continuously from the metal to the insulator.

Laughlin’s group also has done some work on quantum criticality that should be published this year. In collaboration with Jan Zaanen, they have an article on spin-orbit coupling and interesting quantum interference effects that will also be published in this period. The Doniach group expects to continue work on the relation between superconductivity and electron correlations in intercalated graphite. Recent work on superconductivity in doped graphite intercalated compounds has suggested that, despite theoretical work to model this by conventional electron phonon coupling BCS theory, these theories have been unable to explain why different intercalates have very different T_c’s. T_c spans at least one order of magnitude when going through different intercalates and it is believed possible to study the relative effects of t-J correlation coupling versus electron-phonon coupling on these various T_c’s.

The Doniach group has been focusing on looking at electron correlations in graphite. Although this has traditionally been considered in the context of band theory, recent experiments by Kopelevich and collaborators show strong evidence of a semi-metal - insulator transition in applied magnetic fields of order 1 kG. The experimentalists found that a scaling relation suggested by Das and Doniach in 2001 for the superconductor – “bose metal” state of thin filmsuperconductors also applies to the graphite data. The effects of correlations in graphite have been studied using a t-J model, following ideas proposed by Baskaran for the superconductor MgB2.

2. Magnetic Materials Research

J. Stöhr, H. Siegmann, H. Ohldag, Y. Acremann

The general goal of this program is to develop new techniques and approaches for the study of modern magnetic materials in the form of thin films, multilayers and nanostructures and explore the origin of magnetic phenomena associated with such materials.

Work has continued on the phenomenon of exchange bias by means of X-ray microscopy and spectroscopy. In particular, soft X-ray dichroism absorption spectroscopy was used to investigate the direction of interfacial exchange coupling in a ferromagnetic / antiferromagnetic Co/FeF₂ bilayer. This system behaves quite differently from conventional exchange bias and, depending on sample preparation, it can exhibit either positive or negative exchange bias. Two different kinds of interfacial uncompensated Fe moments were found in FeF₂. A smaller pinned portion couples antiparallel to the ferromagnet and can lead to positive or negative exchange bias depending on the size of the cooling field. A larger portion couples more strongly and parallel to the ferromagnet. It increases the degree of antiferromagnetic order in FeF₂ near the interface and plays an important role in the observed coercivity increase at high temperatures. The work was recently published: H. Ohldag, H. Shi, E. Arenholz, D. Lederman and J. Stöhr, Phys. Rev. Lett. 96, 027203 (2006).

The program on ultrafast magnetic switching with electron beams and the program on lensless magnetic imaging with X-rays, reported last years, have now been funded separately as part of the Stanford Ultrafast Science Center (PULSE) and the work is reported under that program.

A new subprogram has started, based on the exploration of new ways of exciting magnetic materials by ultrafast magnetic field pulses. Rather than using electron beams ultrafast laser pulses have been explored for launching fast and strong current pulses. The idea is to generate ultrafast magnetic field pulses by current flow through a strip-line, as illustrated in Figure 1. The current pulse is triggered by an ultrafast laser which either opens an electro-optical (Auston) switch or creates ultrafast photoemission into vacuum on one side of the stripline which is grounded on the other side. Such a scheme has the advantage of being compatible with ultrafast X-ray imaging.

Figure 1. Arrangement for generating short and strong current pulses traveling down a strip line. In the upper picture the current pulse is generated by a photoconductive switch, in the lower picture by a photoemission pulse.

The scheme illustrated in Figure 1 using laser pulse probe experiments will be explored and will characterize the achievable pulse length and field strength. Samples will be fabricated with our new magnetic thin film deposition system and by use of lithographic manufacturing techniques.

The design of a scanning transmission X-ray microscope for BL13 at SSRL will also continue.

3. Scientific and Educational Gateway Program

A. Nilsson, A. Mehta and R. R. Chianelli

This continuing, joint effort with the University of Texas at El Paso (UTEP) serves both the Mexican-American and Mexican communities in undergraduate and graduate education by engaging student scholars in science and engineering research programs at all levels. The program provides travel support for Mexican-American and Mexican students and supporting faculty, science and technological support by an SSRL scientific staff member, who also assists participants in beam line operations and laboratory facilities use, and a scientific staff member at UTEP, who develops and implements computational tools and software for analysis of synchrotron data. These staff members train students in methods of data reduction and analysis and, jointly with SSRL staff scientists, develop collaboratory tools for remote access to instrumentation and data measured at SSRL. This program has been quite effective, as shown by the number of UTEP students participating in FY2001-2005 (almost 50 students). These students and staff underwent training and carried out experiments on existing SSRL peer-reviewed proposals coordinated across five separate beam lines. The students continue to enhance their training by taking highly successful proposal writing classes at UTEP (Chianelli). SSRL staff work closely with the UTEP faculty and staff to train and support the new students and their research efforts. Part of the period in FY2005 saw the synchrotron start-up for the now successfully completed SPEAR3 upgrade. In addition, an electrical accident further delayed the schedule. However, this allowed time for the students to concentrate on data analysis with the help of SSRL and UTEP staff. Progress during this period was excellent with many students and faculty further developing their proficiency in the use of synchrotron techniques and producing publications based on previous collected data. Several students achieved in-depth understanding of synchrotron radiation and its application by attending the Stanford-Berkeley Summer School.

The level of synchrotron related skills increased substantially for both the students and faculty, which will be reflected by refereed publications and meeting presentations of the group. Particularly skills were improved in the areas of SAXS and carbon edge XANES. Progress was made in six core areas in FY2005; (1) The Chianelli group continued to address structure/function relations in transition metal sulfide (TMS) hydrodesulfurization (HDS) nanocatalysts. The first “Gateway” Ph.D. (Myriam Perez) graduated and accepted a post-doctoral position at SSRL; (2) Diffraction studies on Maya Blue demonstrated that the pigment is a surface complex of the indigodye with palygorskite clay; (3) Data analysis on both the SAXS and WAXS data from asphaltenes, crude oils from Mexico and Venezuela and TMS catalysts was completed; (4) The Gardea group continued to investigate the metal binding properties of the “hyperaccumulators”; (5) The Pingitore group continued to study trace elements in human bone. The incorporation of Sr, Zn, Pb, etc. in human bone is a topic that impacts archaeology, nuclear waste/terrorism, biomedicine, and environmental pollution. All of these projects yielded significant publications in 2005 and in press for 2006.

The SPEAR3 installation allowed time to develop the full scientific impact of the program. This is shown by the significant increase in the demonstrated impact of synchrotron studies on the scientific understanding of the problems discussed above. Students and faculty are well trained to participate in “bringing on line” the new SPEAR3 experiments and testing the new significant capabilities.

Approximately 60 students will have been involved in synchrotron programs at SSRL involving multiple techniques, diffraction (SAXS and WAXS), scattering techniques including anomalous dispersion scattering, XAFS (hard and soft X-ray) and protein crystallography. Many of these students will become a regular part of the synchrotron user community and some will have synchrotron use become a major part of their careers. Many these students will have obtained experience at other DOE facilities. Up to four students will have completed or nearly finished a Gateway Ph.D. and become full members of the synchrotron community. Several faculty members at UTEP and related Mexican institutions will have become new synchrotron users. Major scientific progress will have been reported from the Gateway program as discussed above. The progress will be of such quality that the program will not only have made a major impact on training minority scientists but the students will have produced significant and competitive scientific contributions.

4. Novel Materials and Model Systems for the Study of Correlated Phenomena

I.R. Fisher, M.R. Beasley, T.H. Geballe, M. Greven, N. Barisic, G. Chabot-Couture, A.S. Erickson, G. Koster, L. Litvak, Y. Matsushita, N. Ru, P. SanGiorgio, K.Y. Shin, G. Yu, X. Zhao, X. Zhou

This program directly addresses scientific questions at the heart of understanding correlated electron behavior in complex materials. This work explores the conditions of occurrence and mechanisms behind these effects, and examines the consequences for bulk properties and collective phenomena. The approach is to identify and synthesize model systems that enable exploration of particular interactions in isolation. The essence of the proposed research therefore lies in the design, growth and characterization of novel materials.

The original title of this subtask was “Nanoscale ordering in complex oxides: model systems for local probes.” The title was updated during the recent renewal to better reflect the focus of our current research.

Negative-U Impurities --Over the course of the last three years a multistep synthesis route has been established to produce high-quality single crystals of the unusual superconductor (Pb_1-xTl_x)Te, and work has gone forward on detailed measurements to characterize the thermodynamic, transport and spectroscopic properties of these samples. The Tl impurities appear to act as negative-U centers in this material, and this research explores the role that these impurities play in both the superconducting and normal state properties. Early measurements established that a critical concentration of Tl impurities is required to cause superconductivity in PbTe. For Tl concentrations beyond this critical value the Fermi level appears to be pinned, such that the Tl impurities act in a self-compensating manner. It seems that the superconductivity is intimately linked to the presence of a mixed Tl valence, the presence of which also is reflected in the normal state transport of the material. In particular, during FY2005 evidence was found for a charge Kondo effect associated with degenerate valence states of the Tl impurities. This observation, supports the notion of an electronic pairing mechanism in superconducting Tl-doped PbTe, perhaps accounting for the unusually high Tc value. Ongoing research seeks to further investigate this effect in Tl-doped PbTe and related materials, testing various predictions of the theoretical models that have been proposed.

In FY006, the superconducting properties of Tl-doped PbTe have been explored in more detail. Heat capacity measurements for the highest Tl concentrations indicate that the superconducting anomaly is close to the BCS result (ΔC/γT_c ~ 1.43), which within the charge Kondo model would imply that the renormalized electrons are indeed participating in the superconductivity. However, this quantity appears to show a marked decrease as the impurity concentration decreases, the origin of which effect is uncertain and warrants further investigation. Additional insight can be expected from a close collaboration with C. Gough and E. M. Forgan (Birmingham, UK) exploring the penetration depth via microwave conductivity and muon spin rotation. Initial experiments from early this year indicate that the T-dependence of the superfluid density is close to BCS predictions, but with some unusual field dependence. The continuing normal state magnetotransport measurements show an unexpected trend towards a linear T-dependence of the resistivity that we are continuing to study. These data are currently being modeled by J. Schmalian (Ames Laboratory) and P. Coleman (Rutgers) in terms of a pair-diffusion process associated with the negative-U properties of the Tl-impurities. Initial EXAFS measurements (in collaboration with F. Bridges, UCSC) are suggestive of two Tl-Te distances, consistent with the proposed disproportionation, but are limited in resolution due to the proximity of a nearby Pb absorption edge. Experiments are also planned at the NHMFL Microkelvin Laboratory (with Yoon Seuk Lee, U. Florida) to look for reentrant behavior at low temperatures, a key prediction of Schmalian’s charge Kondo model. Additional external collaborations will probe the effect of pressure on the superconducting and normal state properties (S. Brown, UCLA), the optical conductivity (Z. Schlesinger, UCSC) and core level spectroscopy (C. Fadley, UCSD).

Charge-Density Waves in Layered Rare Earth Tellurides -- We recently identified the layered rare earth (R) tellurides RTe3 and RTe2 as model systems to study Fermi surface reconstruction in incommensurate CDW compounds, a topic that has significant bearing on current questions in the field of cuprates and other strongly correlated oxides. These are particularly attractive model systems because (a) the electronic structure is especially simple and the band filling can be tuned by chemical substitution; (b) the CDW gap is large and can be measured by a variety of powerful techniques; (c) we can explore the consequences of the CDW formation on the magnetic properties of the material; and (d) we are able to grow high-quality single crystals, enabling sensitive probes of the Fermi surface including dHvA and ARPES, and real space probes including STM.

Initial angle resolved photoemission experiments in collaboration with Z.X. Shen clearly revealed the CDW gap, showing gapped and ungapped regions of the Fermi surface. Our ongoing experiments further explored the Fermi surface nesting and CDW formation in both RTe3. Further experiments probing the FS reconstruction in RTe3 were conducted by a student supported by thisgrant during an extended visit to A.P. Mackenzie’s research group in St. Andrews, Scotland. These measurements revealed multiple frequencies all of which vary with angle according to 1/cos(theta), consistent with minimal z-axis dispersion. The measurements are currently being compared with ARPES data with the aim of fully determining the reconstructed FS. Other experiments have been performed at SSRL, following award of beam time. Initial high-resolution diffraction experiments have revealed an extremely well-ordered modulated structure with minimal harmonic content. These measurements are continuing with the ultimate goal of finding a model for the atomic displacements in the CDW state, and hence determining whether the material consists of short regions of commensurate modulations with regular discommensurations, or is indeed a truly incommensurate structure.

Experiments at SSRL have established that the CDW in RTe₃ has remarkably little harmonic content. Work is currently in process to model the atomic displacements in an effort to differentiate between a truly incommensurate distortion and a modulation consisting of a dense array of discommensurations. Further diffraction experiments were planned to follow the T-dependence of the CDW wave-vector, motivated in part by theoretical models of S. Kivelson (Stanford) predicting a possible phase transition between unidirectional (stripe) and bidirectional (checkerboard) order. Additional experiments and analysis will finalize our dHvA study, while magnetotransport measurements will pursue our recent observation of superzone gap formation in the heavy rare earth members of the series (i.e. an additional gapping of the FS at T_N). Work has also continued to explore the role of Te vacancies in RTe_2-x in determining the optimal nesting wave-vectors and resulting CDW modulation in this single-layer variant. Most excitingly, there is now confidence the alternating single and double layer compound R2Te5, intermediate between the single layer RTe₂ and bilayer RTe₃ compounds has been successfully synthesized. Experiments are probing how band filling affects the CDW formation in this family of compounds, providing a useful window on the role of dimensionality (stripe vs checkerboard) in CDW systems. Numerous internal and external collaborations continue to probe other physical properties of these materials, including STM (A. Kapitulnik, Stanford), optical conductivity (L. DeGiorgi, ETH, Zürich), NMR (V. Brouet, Université de Paris Sud, France) and positron annihilation (S. Dugdale, Bristol, UK).

Ferromagnetism in the Mott Insulator Ba₂NaOsO₆ -- The interplay between orbital and spin degrees of freedom can play a pivotal role in many correlated electron effects. Perhaps the clearest example of this is to be found in ferromagnetic Mott insulators – a rarified and unusual class of materials. In a single band Hubbard model, hopping between adjacent sites leads to anantiferromagnetic ground state. However, orbital degeneracy can favor ferromagnetic exchange due to the additional energy that can be gained from Hund's rule coupling of electrons in two different orbitals. In such a case, the appearance of ferromagnetism is thought to be accompanied by a complex form of anti-ferro orbital order, the precise nature of which depends strongly on details of the specific lattice and material. This is an exciting and topical area in correlated electron physics. However, there are precious few candidate materials. Furthermore, those that have been studied belong exclusively to the 3d transition series.

Ba₂NaOsO₆ presents a wonderful opportunity to study the interplay of orbital and spin ordering in the unusual environment of a 5d¹ ferromagnet. A method has recently been determined to grow relatively large single crystals of this material that enable a host of exciting experiments. Initial thermodynamic and transport measurements, performed early in FY2006, demonstrate that this material is indeed a ferromagnetic Mott insulator, but with an ordered moment that is substantially less than 1 μ_B (i.e., the ordered state consists of a complicated spin structure with a net ferromagnetic moment). The reason for this is becoming evident. At room temperature, Ba₂NaOsO₆ occupies an undistorted double-perovskite structure, in which the OsO₆ coordination polyhedra are neither rotated with respect to the lattice, nor distorted from a perfect octahedral symmetry. In this case, the three t_2g orbitals are indeed degenerate, and ferromagnetic exchange can be anticipated. However, the FCC lattice cannot be decorated with three "color" orbitals in such a way that each site always has nearest neighbors of a different color. For adjacent sites of the same color the superexchange is perforce antiferromagnetic. Hence, in the absence of any additional structural phase transition, it can be anticipated that the resulting ground state of Ba2NaOsO6 is a complex mixture of spin and orbital order, with a strong ferromagnetic component, but which falls short of 1 μ_B. An active proposal is in place with the Advanced PhotonSource (together with Z. Islam, APS, ANL) to study the spin and orbital ordering in this compound later in the year. Other ongoing experiments will probe the high-field magnetization (with L. Balicas, NHMFL), exploring the possibility of a metamagnetic transition to the fully saturated state.

Electronic Inhomogeneities in Correlated Electron Materials -- One of the goals of this programis to synthesize model systems for the study of nanoscale electronic inhomogeneities that result from the coulomb interactions in correlated electronic materials, and ultimately to study these inhomogeneities using scanning probes. Toward this end, thin films of the Mott insulator CuO have been grown, in which such nanoscale inhomogeneities have been previously reported. This is perhaps the simplest conceivable copper oxide related to the high temperature cuprate superconductors. Epitaxial thin films of stable monoclinic CuO have been grown previously on MgO substrates. More recently CuO has been grown epitaxially with square symmetry in the plane (i.e., either cubic or tetragonal overall). The composition is confirmed by UPS spectra taken in-situ in an adjacent chamber. The structure has been confirmed using in-situ RHEED and byPhoto-Electron Diffraction. Ion-beam assisted deposition (IBAD) has been tried to further enhance our ability to get a cubic structure, but this has not so far been successful, due presumably to differential sputtering of oxygen and cupper. Very recently, it has also shown that CuO can be grown using Pulsed Laser Deposition. The detailed properties of films grown by these two methods now need to be compared.

In parallel with this effort, thin films of ZrO₂ have been deposited using pulsed laser deposition(PLD) for use as high-K dielectrics for field-effect doping applications. The student working on this project has left the university. In light of this, focus will revert to the CuO work and this part of the project will not be continuing.

A square-planar (rock salt) form of CuO has now been deposited successfully and physical study of these films is being pursued. Standard transport and optical measurements are being carried out, and various scanning probes are being applied in collaboration with colleagues here at Stanford. These include Scanning Hall probes, MFM, STM and scanning potentiometry to look for inhomogeneities. In addition, on the materials side, studies will begin on approaches to dope these films. This might be done through introducing oxygen deficiency or by using strongly electropositive (or negative) over-layers.

Superconductivity and Magnetism: Pair Density Waves and SrRuO₃ --Studies of the superconductor/ferromagnetic proximity have drawn a remarkable level of interest, primarily because it is proving to be an effective model system for the study of the interaction between superconductivity and magnetism. One of the most striking predictions from theory is the existence of decaying pair density waves in the ferromagnetic side of an SF proximity bilayer due to the exchange field in the F material. Evidence for this effect is mixed. In order to get more detailed information regarding SF proximity structures, in collaboration with the group of Kookrin Char at Seoul National University, tunneling density of states studies are in progress on the F side of Nb/permalloy SF bilayers as a function of the thickness of the F layer. It is found that the striking result that the tunneling density of states as a function of thickness (viewed as deviations from the normal density of states) is of constant form with amplitude that scales exponentially over four orders of magnitude as a function of the F layer thickness. No evidence for pair density oscillations was found. More recently, a detailed comparison of these results with the available theory has been carried out. It is found that the theory can account for these data only if patently unreasonable values are used of the exchange field and the spin-flip scattering rate. This work has now been accepted for publication. One possible reason for this serious discrepancy is that we are using very strong ferromagnets whereas the theory assumes weak magnetism. In order to clarify the situation, a second strong ferromagnet (Ni) is presently being studied for the F material.

Thin films of SrRuO₃, a rare example of a 4d itinerant ferromagnet continue o be deposited and studied. With the group of Lior Klein, the magneto-transport in SrRuO₃ is being examined to test recent theories of the effect of the Berry phase in magnetic systems. We did not confirm the predictions. Studies also continue of various physical properties of the films including spin accumulation at domain walls during electric transport, the magnetic anisotropy of the material and the superconducting properties of Nb/SrRuO₃ SF hybrids. In SF hybrids there is only magnetic coupling between the S and F layers (i.e. there is no proximity effect coupling).

Given present results on the tunneling density of states on Nb/permalloy SF proximity bilayers, it is very important to study other F materials to see if these results are universal or specific to permalloy. Studies with Ni as the F material were mentioned above. The next obvious step would be to move systematically to lower exchange fields. Following others, the Ni-Cu alloy system will be used. Scanning Hall probe studies of these bilayers will also be carried out to establish directly the orientation and spatial homogeneity (or not) of the magnetization of the F layers.

As was mentioned last year, it is also planned to use the in situ FTIR system to study the optical conductivity of SrRuO3 at high temperatures. Lower temperature measurements carried out by us with Zack Schlesinger at UCSC showed that a coulomb-like pseudo-gap appears to open at low frequencies as temperature increases. This seems to be part and parcel of the bad metal behavior of this material (i.e., increasing resistivity with increasing temperature beyond the Ioffe-Regal limit).Recent theories of highly incoherent electrons using dynamical mean-field theory predict such a gap should open. Clearly these predictions need to be tested.

Advanced Superconductors --The mercury-based materials HgBa₂Ca_n-1O2_n+2+δ are model high-temperature superconductors due to their relatively simple structure, because they appear to be least affected by chemical disorder, and because of their record superconducting transition temperatures (e.g., for n = 3, T_c = 134 K at ambient pressure and 164 K at 31 GPa). Consequently, these materials are the most desirable high-temperature superconductors for experimental study. Comparison with results for lower-T_c materials will eventually allow us to separate materials-specific properties from properties shared by all superconducting cuprates. However, the synthesis of this homologous series has remained a serious challenge until recently, and rather few experiments have been done on the Hg family of superconductors. In a major breakthrough, we succeeded in growing single crystals of Hg1201 (n=1) as large as 50 mm³, more than two orders of magnitude larger than the previous world record. Neutron diffraction (at NIST) and synchrotron X-ray work (at SSRL) confirmed the single-grain nature of our samples, with a typical mosaic of 0.04 degrees for the smaller X-ray samples. The new crystals have allowed us to begin systematic transport, magnetometry, and resonant inelastic X-ray scattering (RIXS) experiments (at the APS), and to initiate numerous collaborations, both at Stanford and elsewhere, with scientists using complementary experimental techniques. For example, optical spectroscopy measurements by Dr. C.C. Homes (BNL) are consistent with a newly-discovered scaling law for the superfluid density. It is noted that these RIXS experiments led to the important discovery of a subtle incident photon energy dependence of the K-edge RIXS cross section, which needs to be explored in future experiments in order to arrive at satisfactory qualitative and quantitative understanding of the charge-transfer excitations in complex oxides. It furthermore led to the surprising observation of a remnant charge-transfer gap in optimally-doped Hg1201.

The superconducting properties of the new Hg1201 (n=1) crystals are being refined further, through a heat treatment in an oxygen atmosphere, in order to obtain very underdoped and also overdoped samples. This long-term project, which also involves transport measurements and magnetometry, is challenging since uniform oxygen control far away from optimal doping is a non-trivial task, especially for large crystals. The initial RIXS work on optimally-doped Hg1201 will be expanded in order to determine that nature of the electron-hole pair excitations in underdoped samples. Furthermore, the growth of the multilayer (n>1) members of the Hg family of superconductors continue to be studied. Due to the high Hg partial pressure during the growth, this task is more challenging than the growth of the n=1 system. This effort will enable increasingly detailed and valuable experiments on the Hg-based model superconductors, such as X-ray and neutron scattering, optical spectroscopy and Raman scattering, ARPES, STM, as well as thermal and charge transport. Quantitative experimental results for the Hg-based family of materials can be expected to serve as benchmarks for tests of theories of high-Tc superconductivity. Efforts will continue to grow sizable crystals of Y-Bi2212 for inelastic neutron scattering and complementary experimental work.

The effects of chemical inhomogeneities were previously investigated in the bismuth-based family of copper oxide superconductors, Bi₂Sr₂Ca_n-1Cu_nO_2n+1+δ. The double-layer variant (n = 2;“Bi2212”) of this homologous series has been of great interest to the ARPES and STM communities, but systematic neutron scattering work has not been possible due to the lack ofsizable crystals. It was found that the maximum attainable value of T_c can be increased to a new record value of 96K for a small amount of Y doping, i.e., Ca-site disorder, which, in effect, leads to a stoichiometric Bi:Sr ratio of 2:2 and hence zero Sr-site disorder. These results have the important implication that the degree and the type of disorder are very important experimental parameters that can and should be controlled: a new generation of experiments on such optimized samples is clearly called for. Efforts have continued to grow sizable crystals for inelastic neutron scattering and complementary experimental work.

5. Nanoscaled Magnetism in the Vortex State of High-Tc Cuprates

S. Zhang, A. Bernevig, T. Hughes, R. Li, X. Qi

This work explores fundamental physical processes which give rise to novel collective phenomena and self-assembled nanostructures resulting from high magnetic fields or complex synthesis processes. The complex nanostructures include antiferromagnetism inside the vortex cores and checkerboard charge order of the Cooper pairs in high T_c superconductors.

A major accomplishment of our research program is the theory of the “pair density wave” (PDW), which describes the checkerboard charge order of Cooper pairs in high Tc superconductors. Due to strong correlations or Fermi surface nesting, electrons can form a self-organized crystal like a Wigner crystal, or a charge-density-wave. In these charge-ordered states, the elementary unit cell contains only a single electron. On the other hand, the PDW state is a charge ordered state of the Cooper pairs, rather than single electrons. We showed that such a state could be favored in underdoped cuprates, where local pairing force is strong, but the kinetic energy of the holes is reduced. A global phase diagram was proposed to describe the competition between the antiferromagnetic state, the d-wave superconducting state and the PDW states. Most remarkably, the PDW state only exists at certain magic, rational doping fractions, e.g. at x=1/8, 1/16, 3/16,where the denominator is a power of 2. At these magic filling fractions, the Cooper pairs form a checkerboard structure.

Some of these theoretical predictions were soon confirmed experimentally. Earlier STM experiments at Stanford and Berkeley have uncovered microscopic charge ordering on the surface of high Tc cuprates. More recent STM experiments unveiled a more precise 4x4 checkerboard charge pattern on the surface of CaNaCuOCl crystals. Close to a doping level x=1/8, the charge ordering pattern is only consistent with the charge ordering of the hole pairs, in accordance with our theoretical proposal. Another remarkable transport experiment was carried out by Ando’s group, in which they systematically measured the resistivity as a function of doping, and identified certain “magic doping fractions” at x=1/16, 3/16, 1/8 and 3/8, where the resistivity displays a maximum, indicating charge ordering tendencies. This observation is again consistent with our predicted global phase diagram.

The work on the pair-density-wave and charge ordering received broad attention in the high T_c community. It was featured in the “News and Views” section of Science, in an article entitled “Crystalline Electron Pairs.” The PI was invited to speak at numerous international conferences, including the Gordon Conference and the Aspen winter conference. The two students working on the projects have successfully completely their Ph.D.s and are continuing their scientific careers through postdoctoral positions elsewhere.

6. Nanoscale Electronic Self-Organization in Complex Oxides

A. Kapitulnik, H. Manoharan, K.A. Moler, Z.-X. Shen, H. Bluhm, A. Fang, A.A. Geraci, C.W. Hicks, L. Mattos, K. Todd

Nanoscale ordering in complex oxides, where the valence electrons self-organize in ways qualitatively different from those of conventional metals and insulators, is one of the most important outstanding problems in physics today. Our research is inherently multi-disciplinary as is presented below.

ARPES Program

The nanoscience funding has enabled leverage of our core program on strongly correlated materials, as the students and postdoctoral associates are taking experimental shifts for each other. In addition to oxide materials, effort continues to explore nanoscale science in related materials. Collaboration has continued with Ian Fisher’s group on charge density wave materials, and effort has been expanded on materials made of carbon nanoclusters.

Electronic Structure of Charge Density Wave State Rare Earth Tellurides – These materials manifest many competing phases with different electrical properties, and are ideal model systems for phase change materials – complex Tellurides used extensively as memory medium. In collaboration with the Fisher group, the ARPES investigation of the Tellurides from the tri-Tellurides to the di-Tellurides has been extended. Through ARPES, insights were gained on the electronic structure and the underlying reason for the charge density wave formation.

Electronic Structure of Solids Made of Carbon Nano-Clusters – We have made good progress towards understanding the electronic structure of molecular solids made of C₆₀. As was reportedbefore, something was discovered that has been speculated for a long time but never observed before: dramatic change in the electronic structure with molecular orientation. In collaboration with the Osterwalder group of the University of Zürich, molecular rotations using photoelectron diffraction has been directly confirmed.

Solids made of carbon nano-clusters such as fullerenes and diamondoids have been studied. Preliminary data have been obtained on the diamondoids. These are interesting carbon nanoclusters with diamond structure but having their dangling bonds satisfied by hydrogen. They have the benefits of both diamond and nanomaterials and thus great potential for applications.

Magnetic Imaging Program

Using novel scanning techniques, studies continued of several high-T_c systems. In particular the study was emphasized of the interplay between magnetism and superconductivity. In addition, it was realized that much insight could be gained by continuing these studies with higher spatial resolution and lower temperatures. Much of FY2006 have therefore been devoted to the development of a He-3 based scanning Hall probe microscope with a new generation of Hall probes with 100 nm spatial resolution. Below are some of our advances in the past year:

The Mesoscopic Magnetic Imaging of Very Underdoped Cuprate – The existence of partial flux quanta was demonstrated, resulting from wandering pancake vortex stacks, in very underdoped YBCO.

Nanoscale Ferromagnetism, Antiferromagnetism, and Superconductivity in ErNi₂B₂C – Hendrik Bluhm from the Moler group in collaboration with Suchitra Sebastian from Ian Fisher’s group studied the interplay of ferromagnetism, antiferromagnetism, and superconductivity in ErNi₂B₂C. The coexistence of these three phases leads to fascinating nanostructured ferromagnetism. The first high-quality local magnetic images were obtained of this nanostructure, reaching two main conclusions. First, the twin boundaries in the antiferromagnetic state strongly pin vortices. This may be a new model system for planar pinning structures. Second, a spontaneous vortex lattice has been theoretically predicted to exist at low temperatures in the ferromagnetic state. It has been demonstrated that it does not.

It was demonstrated that a spontaneous vortex lattice does not exist in ENBC. Instead, we found a nano-scale magnetic texture. Upon completion of our He-3 based scanning Hall probe microscope, we expect to use the dramatically improved spatial resolution to make substantial progress on identifying the nature of this nano-scale magnetic texture, which results from the coexistence of multiple competing phases.

Magnetic Signatures of Time Reversal Symmetry Breaking in Sr₂RuO₄ – Per Bjornsson from the Moler group magnetically imaged the ab-plane surface of single crystals of the unconventional superconductor Sr₂RuO₄, including one sample with an array of micro-holes, using scanning SQUID and Hall probe microscopy in a dilution refrigerator at low applied magnetic fields. The images show dilute trapped vortices, as would be expected in conventional type-II superconductors, and no other magnetic features. No direct signs of the spontaneous magnetization were found that would be expected in a time reversal symmetry breaking (TRSB) superconductor. These measurements set upper limits on the presence of TRSB signatures in this material. Prof. Yoshi Maeno from the University of Kyoto spent three months in our lab collaborating on the further search for TRSB signatures. Characterization of this fascinating material will continue.

High Resolution STM Studies

This year a variable-temperature UHV STM was operated in pursuit of proposal goals. A new method was also developed to apply local stress to materials. Much development time has been spent on integrating these tools, including atomic dosing and manipulation, with the complex materials that are the focus of this research.

Nanoscale Ordering in Correlated Magnetic Materials – Using STM data acquired at IBM, LailaMattos from the Manoharan group in collaboration with theorists Greg Fiete and Barbara Jones analyzed several artificial Kondo lattices and observed a coherence effect in which the Kondo temperature is enhanced in the center of lattices which are resonant with a 2D surface Fermi wavelength. In addition, a correlation hole (the “Kondo hole”) in a central vacancy was demonstrated through this analysis. A manuscript was written and will be submitted to Nature Physics in the first quarter of 2006.

Local Electronic Structure in Novel Superconductors – A new method for applying pressure to thin superconducting samples was developed. In this apparatus, a piezo induces either uniaxial or biaxial stress in the plane of conducting layers of the compound under investigation. Strain gauges confirmed 70% strain transmission from the piezo to the surface of the samples. This method was applied to BSSCO to distort the unit cell with uniaxial compression and attempt to tune across phase boundaries, but pointed to the utility of using thin films rather than single crystals due to strain relaxation through the thickness of the sample. Achievable equivalent pressures were estimated at 30 to 90 atm.

Following an effort to learn how to prepare surfaces, tunneling spectroscopy measurements have begun on PbTe to explore the proposed charge-Kondo ground state in these materials synthesized by the Fisher group. Temperature-dependent spectra will be obtained that aim to get through the Kondo formation regime, but that cannot yet achieve the lowest temperatures (~1 K) necessary for superconductivity. No local tunneling spectroscopy measurements exist for these materials at any temperature; hence a priority has been placed on this achievement.

Piezo stress measurements are now being applied to SrRuO3 and Y₂B₄C₈O_20-x (Y-248) thin films grown by the Geballe group. The Y-248 sample is expected to have a superconducting T_c of ~81K, and an attempt will be made to detect a shift in T_c as stress is applied via our piezo apparatus. The SrRuO₃ sample displays a ferromagnetic phase transition at ~150K, which can be observed as a kink in the resistivity at the transition temperature. Measurements on epitaxially strained SrRuO3 thin films have demonstrated that the T_c of this transition can be lowered to 30K in an 80 Å thick film (on SrTiO₃ substrate). Work will begin to observe and then tune this phase transition via piezo control.

Electronic Structure of Solids Made of Carbon Nano-Clusters – UHV STM imaging and spectroscopy was performed on solids and monolayers formed from a new form of carbon nanoclusters: diamondoids, nanometer-sized diamond molecules. These samples were provided through an agreement with Chevron who discovered them in crude oil. Doped diamondoid crystals and layers have the promise of being interesting electronic materials, and possibly superconductors following C₆₀. Data have been obtained from crystal molecule layers of diamondoids of the first 4 orders—namely 1, 2, 3, and 4 cages per molecule. Local spectroscopy revealed a variation in the band gap, and sensitivity of the electronic structure to the rotational orientation of the molecules. These observations are very interesting in light of analogous measurements of ARPES on C60 by the Shen group. The Shen group is also collaborating with us and measuring ARPES on diamondoid layers. Our STM measurements are now being compiled in a manuscript for submission planned by April 2007.

Preliminary STM measurements on diamondoid solids reveal hints of vibrational structure revealed by inelastic tunneling spectroscopy. This will be investigated as the phonon structure can then be compared to C₆₀ films and crystals and used for future studies of doping, metallicity, and possibly superconductivity.

STS of Ordered Structures on High-T_c Materials

In the past year the STM-STS system upgrade was completed for better stability and noise performance. As a first project to see the impact of the improved system, Bi₂Sr₂CaCu₂O_8+δ single crystals cleaved in UHV were again studied and measured at low temperatures. The study continues of ordered and inhomogeneous structures on BSCCO:2212 and there are plans to extend it to the single layer material BSCO:2201 produced by the group of M. Greven. The study of CDW in tellurides made by the group of I. Fisher will also be carried forward. This project will continue over the next few years.

Ordered Structures in the LDOS of BSCCO – Exploration continued of the newly remodelled system In particular, a very high-resolution study was performed of the gap structure in real space.128x128 pixels on ~60 square samples yielded many interatomic spectroscopic points. Analysis of the coherence peak size which is much more pronounced, suggests that resonances due to bound states near the gap size can contribute to the coherence peak height. To further explore this possibility, very fine energy resolution was achieved by using the ability to minimize the voltage modulation needed to obtain the conductance data, as well as the integration time on the measuring lock-in. The results have been spectacular. Focusing on the superconducting gap, patches were found of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. These spectra were compared with calculations of the local density of states for a simple phenomenological model in which a 2ξ₀x2ξ₀ patch with an enhanced or suppressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap.

Studies of TbTe₃ – In collaboration with Ian Fisher’s group, and as complementary studies to ARPES, high resolution STM studies were performed of TbTe₃ . The data show the topography of a TbTe₃ surface clearly revealing the CDW ordering. Close inspection of the topography indicates that the CDW periodicity is found in either three- or four-row repetition, averaging to a nesting-|q|vector of size 0.29, as is also found in the accompanying Fourier transform.

STS Studies of the CDW State of CeTe₃ – With the improved STM-STS system it is planned to study CeTe₃to complement the initial studies of TbTe₃. Here the gap is expected to be larger (~400 meV) in optimally nested regions of the Fermi Surface (FS), whereas other sections with poorer nesting should remain ungapped. Using this instrument, both the CDW and the signal from the ungapped part of the Fermi surface will be observed.

Optical Tests for Time Reversal Symmetry Breaking in Sr₂RuO₄ – The construction has been completed of the first-ever zero-area fiber optical Sagnac interferometer for measuring absolute magneto-optic Kerr rotation at cryogenic temperatures. A single strand of Polarization-Maintaining fiber is fed into a liquid helium cryogenic probe, eliminating the need for optical viewports and makes the apparatus immune to temperature change. With an optical power of 10μW at 1550 nm wavelength, Kerr measurements are demonstrated on SrRuO₃ thin films with a shot-noise limited sensitivity of 1×10^⁻⁷rad / √Hz from 250K down to 5K without any modulation of the magnetic state of the sample. Typical drift is measured to be 1×10^⁻⁷rad / Hour . This system is ready to be incorporated into a He-3 cryostat for measurements on Sr₂RuO₄.

7. Nano-Magnetism

J. Stöhr, H. Siegmann, Y. Acremann, V. Chembrolu, J. P. Strachan, X. Yu

The general goal of this program is to explore spin currents and the associated quantum mechanical exchange interaction for the excitation and switching of the magnetization in magnetic nanostructures. In particular, this program is based on the use of unique time-dependent X-ray imaging techniques with tens of nanometers spatial and tens of picoseconds temporal resolution.

The manufacture of nanoscale spin injection samples has been accomplished and X-ray imaging techniques have been developed with 30 nm spatial and 100 ps temporal resolution. Using time-dependent scanning transmission X-ray microscopy the changes induced in a buried magnetic sensor layer of 100 x 150 nm² dimension have been directly imaged by spin currents. These results reveal unique new curled magnetization states which arise from the interplay of spin and charge currents, as illustrated in Figure1.

Figure 1. Left: Schematic of the pillar structure, showing the ferromagnetic layers in blue, the antiferromagnetic pinning layer in green and the Cu leads and spacer layers in orange. The bottom two FM layers are coupled into a fixed antiferromagnetic arrangement by a Ru spacer layer and their magnetization direction is pinned by exchange coupling to the green antiferromagnet shown at the bottom. The incident X-ray beam is incident 30⁰ from the surface normal and is focused by a zone plate to a size of about 30 nm. The transmission through the structure as a function of sample position is monitored by an X-ray detector.
Right: Measured time dependent magnetization directions indicated by arrows within the 4 nm thick Co_0.86Fe_0.14 sensor layer inside the 100 nm x 150 nm nanopillar. Images at four times are shown. By varying the delay time of the X-ray probe pulse relative to the current pulse entire motion pictures were recorded.

Motion pictures with 200 picosecond time resolution and 30 nm spatial resolution reveal a fast, subnanosecond switching process based on the lateral displacement of a magnetic vortex, as illustrated at four selected times in Figure 1. While the motion of vortices is omnipresent in nature their role in magnetic switching has previously remained unrecognized or unappreciated. Our measurements show how the injected spin current laterally displaces magnetic vortices created by the curled Oersted field of the accompanying charge current. The new fundamental switching process is intriguing in that it is accelerated by the curly Oersted field yet it may result in metastable final states which are undesirable in technological applications.

Now that the experiment technique has been established and demonstrated different magnetic configurations and sample geometries will be explored. Of particular interest are sample geometries where the electrical part of the current does not directly flow through the sensor layer but the effect of only the spin polarized electrons that diffuse into the sensor layer can be studied. This will be explored with new types of samples which do not have the form of a pillar but are planar. One of the main questions is whether the switching time in such “spin-current only structures” will be decreased by the absence of the Oersted field which accompanies the charge current.

8. Behavior of Charges, Excitons and Plasmons at Organic/Inorganic Interfaces

M.D. McGehee, N. Melosh, M. Brongersma

As electronic device dimensions shrink to nanometer scales and the range of desirable applications grows, two trends are emerging. First, the range of materials under serious development is growing, and many device structures consist of both organic and inorganic building blocks. Second, many physical phenomena that were heretofore only observed within academic experiments are becoming important for technologically relevant devices. Consequently, there are a large number of technical issues that need to be solved before these new possibilities become technologically viable. These include reproducible device performance on this length scale, sample heterogeneity, interface state control, defect properties, thermal transport and surface roughness. In addition, physical phenomena such as electron tunneling, Förster coupling, and plasmon-excitation quenching begin to severely impact device behavior at length scales less than 10 nm. This is particularly true within the emerging subset of structures that utilize both organic and inorganic materials, such as solar cells, electronic paper, molecular electronics, and organic light emitting displays. Within this rather broad collection of challenges, our team has identified the need to understand excited state behavior within organic species close to inorganic surfaces as a key problem for future applications of these materials.

Excited state phenomena within organic materials are often complicated by the multiple length scales, morphology, multiple competing decay processes, and inorganic surface interactions that affect the overall behavior of the system. Current studies of realistic devices are complicated by simultaneous excitation decay via a number of different processes within different regions of the sample. These decay processes, in which the charge or energy of an excited state in a molecule is transferred to an adjacent metal electrode, depend strongly upon the molecule to metal spacing.

In order to address these issues systematically, our team examines exciton transfer and decay within organic systems on a hierarchy of length scales. Melosh studies exciton transport and molecular wave function coupling to metal electrodes on the molecular level (~1 nm). McGehee examines exciton and charge transport within conducting polymer films close to metallic electrodes or dielectric films (5-100 nm). Brongersma investigates how excitons couple to surface plasmon waves on metal surfaces within the 10-500 nm range. Collectively, these measurements provide a better overall understanding of the behavior and importance of charges, excitons and plasmons within electrically active organic-inorganics than would be possible from a single study alone.

Charge Transport in Conjugated Polymers: The structure has been characterized and the charge carrier mobility measured of films of regioregular poly (3-hexylthiophene) P3HT as a function ofmolecular weight, casting solvent, annealing temperature and surface treatment. The primary characterization technique was X-ray diffraction at SSRL. It has been seen that when low molecular weight polymers and low boiling point solvents are used, crystals form rapidly in the bulk of the film. Consequently the crystals do not align with each other and the conjugated parts of the polymer chains do not stack up against each other at the grain boundaries. The charge carrier mobility can be as low as 10^-6 cm²/Vs. On the other hand, when higher molecular weight polymer and higher boiling point solvents are used, crystallization tends to occur only at the surface of the gate dielectric. Consequently, the crystals all tend to have the same orientation and charge carriers can easily hop from one crystal to another. The mobility in this case can be as high as 10^-1 cm²/Vs.These studies highlight how critical it is to optimize the structure of organic semiconductor films when assessing new molecules for applications. The initial demonstration used P3HT as the light absorbing and energy donating material. This polymer is actually a poor choice because it is not a good light emitter and therefore is not a good energy donor by the Förster mechanism. Better emitters have been used and it was found that energy transfer becomes effective over distances as large as 30 nm. Unfortunately, the energy levels of the polymer used were not suitable for splitting excitons at the interface between the two polymers. A goal for this year is to obtain suitable polymers from the Molecular Foundry and Jean Fréchet’s group at UC Berkeley so that solar cells that utilize energy transfer over 30 nm distances can be made.

Exciton Transport in Conjugated Polymers: Almost all organic photovoltaic cells are based on either planar or bulk heterojunctions of two semiconductors. After light is absorbed, excitons must get to the interface between the two semiconductors to dissociate by electron transfer. In some cases, such as in dye-sensitized cells or polymer-fullerene bulk heterojunctions with very high fullerene concentrations, excitons are formed right at the interface and exciton transport is therefore not a limiting factor on the performance of the cells. In many other cases, such as in polymer-nanowire or polymer-titania cells, excitons need to travel at least five nanometers, if not more. For this reason exciton diffusion is a very important process to understand and optimize. Exciton diffusion must also be avoided in light-emitting diodes so that excitons do not reach quenching sites. The exciton diffusion length has been measured in several polymers and it was found that the values are less than reported in the literature. Common sources of error in diffusion length measurements are neglecting interdiffusion between the donor and acceptor, interference effects and resonance energy transfer. Since the diffusion length in most polymers is 6 nm or less, ways have been explored to enhance exciton transport. One is to use resonance energy transfer from a donor to an acceptor with a slightly smaller energy gap. It has been shown that exciton harvesting form P3HT, a polymer commonly used in the best polymer solar cells, can be improved by a factor of three by transferring energy to a low band gap polymer. This improvement in exciton harvesting triples the efficiency of solar cells made with P3HT. It has also been shown that resonance energy transfer occurs in many previously studied donor-acceptor blends, including polymer-fullerene blends with low fullerene concentrations.

Determining the Orientation of Organic Molecules at Buried Interfaces: It has recently beenshown that crystals nucleate at the interface with the dielectric in polymer thin film transistors by analyzing XRD rocking curves and observing that the crystals could only be as well oriented as they are if they nucleate off of the interface. This technique will be used to address the general question of how conjugated polymers pack at interfaces. This topic is particularly relevant to organic-inorganic solar cells. It is hypothesized that in some cases polymers, such as P3HT, pack with their side chains pointing towards a titania surface. For reasons not yet understood, in other cases the polymer packs with its conjugated backbone attached to the titania. It is thought that the cells with the backbone attached to the titania work much better because there is no barrier to electron transfer.

Omni-Directional Emitters and Plasmon Coupling: Brongersma demonstrated in a series of experiments and electromagnetic simulations that metal/light emitting molecule/metal optical microcavities can be engineered to produce an omnidirectional emission resonance. The two metal layers in this cavity have a clear optical function, but at the same time they also serve as electrical contacts for current injection. This is quite desirable from an electronics viewpoint as metals exhibit a low resistance. However, in current light emitting devices metals are generally avoided and more resistive transparent conductors such as Indium Tin Oxide are used as they allow for more efficient light extraction. Brongersma is interested in further exploring the use of metals in light emitters and exploiting the unique properties of surface plasmon-polaritons to increase the out-coupling efficiency, decrease the lifetime of emitters, and modify the angular emission distribution. In this study, he showed that for a cavity thickness equal to one-quarter of the surface plasmon resonance wavelength, λ_sp/4, a completely flat dispersion relation for surface plasmonpolaritons can be realized. Dipolar emitters placed in such a cavity radiatively decay into surface plasmon-polariton modes that subsequently couple to far-field radiation in all directions. This omni-directional resonance contrasts sharply with the typically highly directional resonant enhancements from waveguided modes in planar dielectric optical microcavities. In order to experimentally verify the predicted behavior, the angular emission from a blue-emitting polymer within a gold cavity was measured. In these experiments the omni-directional behavior was indeed observed and the work is currently being submitted to Applied Physics Letters. These studies may be of importance for the field of solid state light emission, which is rapidly gaining in importance in a number of applications such as traffic lights, room lighting, and displays. In particular, the observed isotropic emission is important in applications where a large viewing angle is desired.

Surface Plasmon Spectroscopy: The Melosh group has focused on investigating the optical properties of molecular films within metal-molecule-metal junctions and their behavior during electrical cycling. The goal is to resolve the dispute about molecular re-arrangements under high bias that may lead to different molecular conductivity, which has special importance for molecular electronics and understanding electron transfer near metal electrodes. A new technique has been developed, ‘Surface plasmon spectroscopy’ (SPS) to probe the optical absorption of molecular layers between metal electrodes. This method enables us to ‘see’ inside a 1-3 nm thick molecular film between two opaque metal electrodes. Based upon the Kretschman surface plasmon excitation architecture, this method relies upon the reflectivity change due to coupling into surface plasmons to measure the real and imaginary parts of the molecular film’s dielectric constant. By repeating this measurement at multiple wavelengths the optical absorption spectrum can be obtained from the imaginary component of the dielectric constant.

It was discovered that the absorption of molecules on a single metal surface is almost identical to solution phase, however thin organic films placed between metal electrodes exhibit significant absorption maxima shifts (15-30 nm). These shifts are believed to be due to Stark effects within the junctions, and need to be included in electron transport calculations. Typically these junctions consist of an Au or Al bottom electrode 20-30 nm thick, a thin molecular layer 5-30 nm thick, anda top Au metal contact 15-30 nm thick. However, to simultaneously test the electrical response and optical absorptivity, new top metal contact methods are necessary to prevent pin-hole shorts through the molecular layer.

The limits of Surface Plasmon Spectroscopy will be pushed further in order to determine its maximum sensitivity and resolution. Measurements will be extended to include single molecular thick films of dye molecules R6G and ‘active’ molecules such as spiropyrans and [2]-rotaxanes. These active molecules change their optical, and presumably electronic, state upon either UV absorption or electrical redox chemistry, respectively. Studying the molecular reconfigurations using SPS within these systems will help us understand the nature of molecular electronic behavior in direct contact with electrodes, an important topic for emerging applications utilizing organic species together with inorganic structures.

In addition to direct optical studies, the change in optical properties will be examined as electrical bias is applied to metal-molecule-metal junctions. Molecules may include model systems such as alkanes, or conjugated systems like oligo-phenylethylene vinylene (OPEs). In order to take full advantage of these molecules, the PALO technique must be extended to be able to deposit the top float-on electrode onto a Au bottom electrode, which currently results in shorting. We believe this is due to the high mobility of the Au atoms under large applied bias, thus we are going to investigate methods to lift on Al top electrodes onto Au bottom electrodes.

Surface Plasmon Sources: Brongersma will continue his research in the area of Metal-Molecule-Metal junctions. In addition to using these junctions as light sources, he is interested in exploring the possibility to fabricate a surface plasmon-polariton source. To this end, new simulation tools will be developed that can quantify the emission out of the side of the junction and into surface plasmon-polariton modes. The Metal-Molecule-Metal junctions will be attached to a metal (plasmonic) waveguide that can guide the surface plasmon-polaritons over short (~ 10 μm)distances. Interestingly, the metal waveguide can again serve a dual purpose and act as the electrical contact to the source as well. The end goal is to fabricate a stable, low-noise, high intensity, and compact plasmonic source. These type of pigtailed sources could be of practical importance and can be used to ensure efficient coupling from light sources to optical waveguides and fibers.

Soft Contact Metal Deposition: In order to address electrical shorting in our evaporated metal devices, a new technique was developed to softly deposit the top metal contacts onto an organic film. Depositing the top metal contact for molecular electronics is always difficult, especially for near-atomically flat films. Sputter or e-beam deposited metals always damage the organic sample to some extent, while other ‘soft’ methods like lift-off float-on electrodes wrinkle excessively and stamped contacts are atomically quite rough. Our procedure, “Polymer Assisted Lift Off” (PALO), utilized a thin polymer as a backing to support pre-evaporated metal electrodes, which gave them mechanical strength and hydrophobicity. Using a nonadhesive metal deposited onto an ultra-flat substrate (mica or Si), the electrodes could be ‘lifted-off’ onto a water surface due to the hydrophobicity of the polymer (e.g, PMMA) layer. This floating set of electrodes could then be transferred onto a thin organic film without damage. A paper on the PALO method is under review by Advanced Materials.

Large electrode arrays were created with the PALO technique, with wire widths as small as 1 micron to as large as 3 mm, with lengths up to 3 cm. This is an extraordinary dimension for 20 nm thick metal films, (an aspect ration of 1.5 million!) especially without any wrinkling or buckling of the metal. This is a result of the thermodynamic driving force of the polymer layer to force the water from between the two surfaces, and the favorable maximization of the metal –water surface area. To demonstrate the utility of this technique, we performed studies of electron tunneling through carboxy alkanes with a PALO top contact. These devices demonstrated the same electron tunneling behavior as literature STM and nanopore measurements. Even with 4 mm² contact areas, no shorting is observed.

9. Development and Mechanistic Characterization of Alloy Fuel Cell Catalysts

A. Nilsson, P. Strasser, H. Ogasawara

The main focus of this research program is the investigation of mechanistic aspects of fuel cell catalysis on metal surfaces. One of the main challenges for the Hydrogen Fuel Initiative is to develop cost efficient electrocatalysts with high durability for the next generation of fuel cells. An essential aspect of this project is to develop synchrotron radiation based X-ray diffraction and spectroscopy methods that allow in-situ probing of the intermediates in the catalytic cathode process where both species identification, geometric and electronic structure properties is fully characterized. In parallel to the fundamental synchrotron work, theory-guided combinatorial synthesis and high throughput electrochemical screening methodologies for fuel cell cathode catalysts will be developed and applied in order to link mechanistic hypotheses and catalyst testing under realistic conditions in high dimensional compositional and process parameter spaces.

Oxygen Reduction by Water on Surfaces: The adsorption of water on oxygen covered metal surfaces reduces oxygen to OH forming a OH-water co-adsorbed phase. This OH-water coadsorbed phase is an important intermediate in fuel cell catalysis. XAS, XPS and STM studies were performed on the OH-water co-adsorbed phase on Pt(111) and Ru(001) surfaces. While the OH species is fully hydrogen bonded to surrounding water molecules on Pt(111), the OH species on Ru(001) is in a non-donor broken hydrogen bonded configuration.

Wetting at Water on Surfaces: The fuel cell reaction occurs in confined spatial regions called triple phase boundaries where the gas, electrolyte containing water and catalytic metal particle contact. A wetting behavior of metal surface plays an important role in how triple phase boundaries. We demonstrated on Pt and Cu surfaces that the wetting is related to the difference in substrate electronic structure.

Instrument Development: An electron spectrometer equipped with three differential pumping stages was installed on surface science end-station at SSRL BL5-1 for the in-situ investigation offuel cell reaction. A differentially pumped ambient-pressure reaction cell using cryogenic technology is being assembled.

Structural Molecular Biology

The primary purpose of work described here is to develop synchrotron radiation facilities and provide access for the national scientific community through a strong user support program. Such synchrotron resources are a powerful and versatile tool for research in structural molecular biology, and provide tools very relevant to addressing the U.S. Department of Energy mission needs. The scientific and technological focus of this program includes the applications of synchrotron radiation to macromolecular crystallography, small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS). These efforts are led at SSRL by Professors K.O. Hodgson, B. Hedman and W.I. Weis, and Drs. S.M. Soltis and H. Tsuruta.

Key aspects of the program being provided by the BER funding include:

Continued availability to, and support of users on, state-of-the-art beam lines and instrumentation on the upgraded 3^rd-generation SPEAR3 storage ring for SMB research for a significant fraction of a given year (~9 months or more per year)).
Enhanced user support and training for SMB scientists using up to 10 existing stations at SSRL (of which eight are on high-intensity, multipole wiggler beam lines).
Full operation and user research program on all three stations on the Beam Line 9 facility dedicated to SMB research.
Continued development and implementation of advanced optics, experimental facilities, detectors, computer resources and software to enable optimal advantage to SMB users of the capabilities of the new 3^rd generation SPEAR3 storage ring.
Continuation of capital improvement projects in areas such as beam line enhancements, data acquisition systems including detectors, electronics, controls and computer hardware for SMB stations.
Continued synergistic research and user support in the SMB area with the NIH National Center for Research Resources (NCRR)-funded Biomedical Technology Program (BTP) and the National Institute of General Medical Sciences (NIGMS)-funded macromolecular crystallography operations support and Structure Determination Core of the Joint Center for Structural Genomics.

BER Funded Staffing and New Opportunities for SMB R&D

The BER-funded scientific and technical staff at SSRL currently effectively support users of up to ten existing stations, including the three BER-funded Beam Line 9 SMB stations (BL9-1 and BL92 for macromolecular crystallography and BL9-3 for X-ray absorption spectroscopy). This is done in coordination with other specialized activities supported by NIH NCRR and NIH NIGMS. Ph.D.level research staff, currently all or in part supported by BER, are A. Cohen, R.P. Phizackerley, C. Smith, S.M. Soltis (in crystallography), S. DeBeer George, B. Hedman, K.O. Hodgson (in X-ray absorption spectroscopy), and H. Tsuruta (in small-angle X-ray scattering). Support is also continued for one summer month of salary support for Professor W.I. Weis (a term member of the SSRL faculty with primary appointment in the Department of Structural Biology on Stanford campus; a leading expert in multiple-wavelength anomalous dispersion (MAD) phasing who contributes significantly to continued developments and applications in this important area).

Professor A. Brünger has a joint appointment between SSRL (⅓) and the Stanford School of Medicine (⅔). His activities focus on computational and methodological macromolecular crystallography, and are having a very positive impact on providing new capabilities for SSRL users and staff. Funding continued in FY2005 and FY2006 for support of a graduate student research assistant (S. Kaiser) to work with Dr. Brünger on SSRL SMB-related developments.

Five-Year SMB Program Plan for Beam Line and Instrumentation Developments and for User Operations Support – A competitive five-year renewal proposal for the DOE-BER and NIH-NCRR funded SMB Resource at SSRL was submitted formally to NIH on June 1, 2004. As per discussion and agreement between the DOE-BER and NIH-NCRR program staff, this renewal formed the basis for a joint evaluation of the synergistically funded and managed SMB program at SSRL. The proposal contained developments directed specifically to the BER-funded program, which focuses mainly on developments and implementation of new instrumentation and beam line facilities to enable the SMB user community to benefit in the most optimum way from the new capabilities provided by the new SPEAR3 accelerator. The Special Study Section Committee, which performed a site visit at SSRL in November 2004, provided an exceptionally strong endorsement of the SSRL SMB program as a whole, and recommended that the BER funds requested for personnel, travel, materials and supplies and other non-equipment expenses, be provided at the budget level requested in the proposal. For equipment, it was recommended that all proposed items in the BER budget be funded with the one modification: a reduction in scope of a data storage system, from 300 TB to 100 TB. A FWP, modified according to the Study Section Committee recommendations and with a revised budget, was submitted to DOE-BER on 4/25/05. The proposed Operations Funding and Capital Equipment Funding in this FWP have since been revised and is consistent with programmatic guidance. The NIH-NCRR grant award for the five-year period of the SSRL SMB program has been made.

Structural Molecular Biology Program at SSRL

During the FY2006 run, the SPEAR3 accelerator continued to be exceptionally reliable, providing very stable beam for a very high fraction (96.2%) of the scheduled time, at up to 60+ hr life times. The new rapid fills, taking 1-3 mins each, became routine and greatly enhanced the use of beamtime and the thermal stability of the optics. The FY2006 user run extended over a ~8 month period from ~November 28 through August 7, 2065. Outstanding utilization and performance were seen over the run period. About 50% of the proposals assigned time was for structural molecular biology-related research. Demand for beam time continues to be greater than the available beamtime.

SSRL is planning a normal ~9 month/year run cycle, delivering (as do the other three DOE synchrotrons) about 5,000 user hours per year (i.e., about 630 scheduled user shifts per year).SSRL has the goal of maintaining or increasing this by as much as 10% over the coming 3-5 years, paced by the needs for operational shutdowns for new beam line insertion devices and other planned upgrades and the availability of adequate base facility operating budgets from DOE BES (which are anticipated given the budget outlook for DOE-SC in the coming 10 years).

The strong demand for all the stations at SSRL continues. Over all the stations at SSRL, the overdemand averaged ~160%. Among the most severe in the overdemand category remain the crystallography stations, with a combined overdemand of ~230% for beam lines BL9-1, BL9-2 and BL11-1. The SAXS wiggler station BL4-2 was in overdemand by 176% despite having become a dedicated beam line for this technique and science, whereas BL9-3 (for XAS) was in overdemand by 267%, one of the highest in all of SSRL. The fraction of beam time on X-ray stations allocated to structural molecular biology research continues to be ~35-40% while the fraction of SMB users is ~45%. Since the majority of this beam time is awarded on a peer-review competitive basis among all SSRL proposals, SMB proposals continue to compete very well at SSRL.

User Satisfaction – As continuing part of user activities at SSRL, each user group is asked at the end of their run to complete an "End-of-run summary form". This form provides an opportunity for pointing out specific problems/issues and offering suggestions as to means of improvement. It also asks several questions to get a reading on overall satisfaction. The forms are analyzed by the SSRL staff and summaries by the SSRL Users Organization Executive Committee. The users rate in five categories (Unsatisfactory to Excellent). In the area of "Overall Scientific Experience" 24% overall were very good and 68% were excellent in FY2005 the FY2006 statistics are being compiled as of the writing of this document. These summaries included all the open SMB beamlines and users and, while subjective, indicate a significant measure of user satisfaction with the operation and service of the facility.

Beam Line 9 Upgrade Project – At the end of the FY2005 run, there remained, to replace the BL9-1 and BL9-2 monochromators as well as about 80% of the beam line masks, slits, filters, and windows. Additionally, motivated by the expected improvement in beam focus, the BL9-2 branch line needed to be realigned 0.4 mrad to the insertion device centerline. At the beginning of the 2005 shutdown, the initial focus was the installation, alignment, plumbing, cabling, and motion control tests of the in-alcove hardware. The BL9 in-alcove installation was completed successfully by November 2005, when attention turned to the out-of-alcove hardware installation. The BL9-1 and BL9-2 monochromators and the rest of the branch line hardware components for all three lines were installed and alignment was completed by January 2006. The low-conductivity water (LCW) system and associated machine protection system (MPS) sensors were replaced. Branch line commissioning began in late January. Staggered starts of user operations of all three branch lines (in order BLs 9-3, 9-2 and 9-1) took place during the month of February, and they are now in full use by the general user community. The upgraded optical components have already led to a demonstrated increased performance of 2X and 3X for BL9-1 and 9-2, respectively, over earlier operating conditions with SPEAR3. All branch lines for BL9 are now ready for SPEAR3 500-mAoperations, which are anticipated to commence during specific periods in the next year’s run cycle.

Beam Line 4 Upgrade Project – The BL4 500-mA upgrade project is yet to be completed due to overall budget constraints. Despite the comparative lack of resources (due mainly to the constrained DOE BES core budget that is sharing the upgraded cost of the BL and the significant cut in those funds in FY2006), many BL4 components including the LN2-cooled monochromators, the monochromator slits, the graphite filters, the Be windows, and the branch line beam stoppers are in various stages of fabrication and assembly or are complete awaiting installation. In the coming months the mirror systems, mirror slits, fan allocation masks, pivot masks, and hutches will become the major focus of the BL4 upgrade effort.

One major and unexpected development in the BL4 upgrade project was the decision in FY2005 to relocate BL4 from its current location in Building 131 to adjacent to BL11 in Building 130.

This decision rests on several factors: (a) Rebuilding BL4 in a new location permits installation activities to commence while BL4-2 continues to serve the Bio-SAXS user community. This will minimize the BL4 down time since the existing BL4 need not be decommissioned until much of the new BL4 is already installed. (The relocated and upgraded BL4 will reuse only the existing BL4 insertion device, front end, hutch instrumentation, and much of the instrumentation and control suite.) (b) The new BL4 location incorporates a portion of the SPEAR3 concrete shield wall that already has been upgraded to current seismic standards. In contrast, the SPEAR3 concrete shield wall at the old BL4 location was scheduled for seismic retrofit in several years. Since the retrofit of this wall necessitates removal of nearby beam line hardware, relocation ofBL4 avoids future disruptions of the beam line during this seismic retrofit. (c) The relatively unencumbered geometry of the new BL4 location simplifies the installation while affording the opportunity to optimize the beam line layout for improved performance, functionality, and user ergonomics. BL4-2 will continue in its new location to serve at 100% the general user community in the area of biological small-angle X-ray scattering/diffraction. Installation and recommissioning of the new BL is anticipated to be complete in FY2007.

Beam Line 7 Upgrade Project – The end of FY2005 and the beginning of 2006 saw a flurry of activity and associated progress on the BL7 500-mA upgrade project such that the beam line at this point is a few working weeks away from the start of commissioning. Much of early FY2005 was spent assembling the remaining BL7 masks and optical systems in preparation for installation. The masks and mirrors to be installed inside the SPEAR3 shielding enclosure received the highest priority for attention to ensure that all such components were ready for installation during the tight summer 2005 installation envelope. The optical configuration of each branch line employs an in-alcove M0 mirror for power filtering, harmonic rejection, and beam vertical focusing (BL7-1) or beam collimation (BL7-3). These single crystal silicon mirrors are side cooled through contact to water-cooled copper pads using a gallium-indium eutectic as a thermal transfer medium. During this same time period, the remaining “out-of-alcove” optical elements, masks, slits, windows, and vacuum transport systems were assembled. Several key elements, such as the BL7-3 LN₂-cooled monochromator, were assembled in prior years and stored for later installation. Among the key components assembled in FY2005 was the BL7-1 side scattering monochromator. This focusing monochromator employs a side-cooled, cube-root cut Si crystal assembly, like BL9-1 and BL11-1.

By the start of the SPEAR3 summer shutdown in August the vacuum assembly of the beam line optics and beam transport components was largely complete and the effort turned to the rapid disassembly and removal of the obsolete BL7 components and radiation enclosures (hutches). Within approximately one month from the start of the shutdown, the site of BL7 was stripped bare of virtually all optics, support hardware, and hutches, and this removal of the old hardware paved the way for installation of the new beam line. The most important installation milestone was the completion of the installation, alignment, plumbing, cabling, and motion control tests of the in-alcove optics and beam transport hardware prior to the close up and restart of SPEAR3 at the beginning of November 2006. While the in-alcove components were being installed, new radiation containment hutches were constructed in the out-of-alcove area. This task is nearing completion atthis point, and the installation of the out-of-alcove beam line optics and beam transport is near completion. When complete, this rebuild will provide 100% general user access to BL7-1 for macromolecular crystallography, with the same infrastructure as provided for all other MC beamlines at SSRL, and BL7-3 will be dedicated to dilute solution biological X-ray absorption spectroscopy (100% general user), alleviating some of the oversubscription of BL9-3 and thereby providing more time for other techniques, such as microXAS imaging and single crystal XAS on BL9-3. The upgrade of BL’s 7-1 and 7-3 is mainly funded from NIH NCRR.

Macromolecular Crystallography

For macromolecular crystallography, advances in instrumentation and beam line technologies continued in FY2006. During the FY2006 run, five MC beam lines were in operation with the new SPEAR3 lattice running at a current of 100 mA (BL11-1, BL9-2, BL9-1, BL1-5 and 50% of the available time on the shared station, BL11-3). The Stanford Auto-Mounting (SAM) system was available on these beam lines. The SAM system incorporates a robot that can mount up to 288 samples without the user having to enter the experimental hutch and operates in an integrated environment within the Blu-Ice experiment control software that can be used to select and screen samples in a totally automated fashion. During FY2006, the SAM system was used routinely by a significant percentage of the user community. A high priority will be placed on optimizing the beam line in-hutch instrumentation for 500-mA SPEAR3 performance. Specifically, the newly rebuilt BL7-1 is being commissioned and made operational for the general user community (and as mentioned – this phase took place for BL9-1 and BL9-2 early in FY2006).

Beam Line 12 – New Capacity and New Experiments – The California Institute of Technology received a $12.7M gift from the Gordon and Betty Moore Foundation to fund the construction of a new high-intensity, state-of-the-art synchrotron beam line for MC research at SSRL. Under a cooperative research agreement, the use of the beam line will be shared between macromolecular crystallography general user community (60%) and scientists at the California Institute of Technology (40%). BL12 will make use of a powerful in-vacuum hybrid undulator source. The 67-period undulator with a 6 mm gap operating on SPEAR3 will produce a brightness of ~10¹⁸p/s/mrad²/mm²/0.1%bp at an energy of 12 keV, approaching that of the APS UA undulator. The beam line will provide a very narrow energy band pass for optimized MAD experiments and is being designed to accommodate the study of microcrystals. With this very bright source, it is projected that samples with dimensions on the order of ~5-10 µm on an edge, and otherwise weakly diffracting crystals, will be able to be routinely studied.

The design and construction of BL12 is progressing well, with commissioning currently scheduled for early FY2007. The undulator ID was delivered in late June and has been installed in SPEAR3. The four dipole magnets required to produce the SPEAR3 orbit chicane for the new ID and the associated new SPEAR3 vacuum chamber have been installed. The quadrupole magnet triplet required to focus the SPEAR3 beam into a double waist for the BL12 ID was fabricated and installed during the summer 2005 SPEAR3 shutdown. The new SPEAR3 magnetic lattice incorporating the double waist was commissioned at 100 mA during the fall. More recently this lattice was successfully injected and run at 500 mA. The beam line front end has been installed. The three mirrors required for the beam line optical concept are currently being installed. The LN2-cooled monochromator hardware has been installed and testing will commence shortly. Modifications to Building 131 and the SPEAR3 concrete shielding enclosure required to extract the beam were initiated and completed during the summer 2005 SPEAR3 shutdown. Overall, the beam transport systems are on schedule for completion by November 2006.

The hutch instrumentation is conceptually established, with the standard SSRL crystallography equipment and software being utilized wherever possible. The crystal mounting robot, cryostreamand fluorescence detector systems will be replicated with some improvements. Modifications to the sample table and detector positioner to increase the stability of the sample handling system are being developed. The designs for systems needed for the visualization and manipulation of micro-samples have also been developed and are based on currently available technologies. A Mar325 CCD detector has been ordered for the beam line. BL12 will provide a state-of-the-art facility for SSRL’s general users to carry out extremely rapid data collection and to routinely study challenging systems such as large complexes, viruses, micro-crystals, and weakly diffracting samples with large unit cells. Funding for general user operation for this beam line was included in the submission (and recommended) of the five-year peer-reviewed proposal to DOE BER, but has not yet been realized. It is our hope that funding for this purpose can be added beyond inflationary increases in the operations budget in FY2007.

Remote Data Collection – Since June 2005, macromolecular crystallography users have had the option to conduct remote user access data collection diffraction experiments from their home institutions and other remote locations by means of advanced software tools that enable control of the beam lines. Remote experimenters have access to the same tools as local users, and have the capability to mount, center, and screen crystalline samples, and to collect, analyze, and backup diffraction data. Automated sample mounting is accomplished with the SAM system. Beam line and experimental control is carried out using Blu-Ice/DCS and additional remote monitoring of the experiment and data backup is supported with several web-based applications. The highly graphical applications and computational resources at SSRL are accessed through a client/server application that uses minimal resources on the client side and has a typical response close to that obtained at the beam line. This remote capability is now available on beam lines 1-5, 9-1, 9-2, 111, and 11-3, and starting in FY2006, 7-1.

Many of the applications required to perform a crystallography experiment are highly graphical in nature, and typically do not perform well through standard remote login techniques over large geographical distances. This problem has been addressed with the use of a terminal server application provided by NoMachine. This server is accessible to remote users through a free client application that is easily installed on their home computers. This system enables the user to run all command line and X-Window based applications available at the beam lines, including the Blu-Ice control software and data processing suites. SAM cassettes with preloaded samples are shipped to SSRL and a staff member loads up to three cassettes into the dispensing dewar at the assigned beam line and authorizes the user group to access the control system in a secure mode. Through a NX client remote desktop, the experimenter selects and mounts the samples from inside a cassette using the Blu-Ice software to access the SAM system. After a sample is mounted, it may be centered in the X-ray beam automatically, or if desired, the experimenter may manually adjust thecrystal position by clicking on the video image, displayed at the home institution, of the sample within Blu-Ice. The SAM system can be configured for fully automated operation to screen large numbers of crystals while unattended. From within Blu-Ice, multiple samples can be selected for screening, and parameters for acquiring test diffraction data can be configured. An automated screening sequence is typically comprised of mounting the sample, aligning the sample loop to the X-ray beam, taking a diffraction image and video image at a phi angle of 0 and 90 degrees, and returning the sample into the cassette. This sequence is repeated for all selected samples. Automated scoring of the diffraction quality of each sample can be carried out. The scoring results are written to a spreadsheet to assist the user in selecting the best sample for data collection. The user can then start data collection on the best crystal, or a series of ranked crystals.

Users are becoming increasingly interested in operating the beam line and accessing computational resources remotely. Out of 65 beam time requests that were received during the June and July scheduling period, there were 18 requests (27%) for partial or full remote access. For the first FY2006 scheduling period, 25 requests (46%) for remote access have been received. To date, remote access has been used for more than 35 experimental user runs representing 20 research groups as far away as Australia and New Zealand. During the commissioning phase, NIAID members on a site visit at The Scripps Research Institute in La Jolla observed the remote screening of ~150 SARS related protein crystals on BL11-1. This screening run aided in the structure determination of a conserved domain from the severe acute respiratory syndrome coronavirus [K.S. Saikatendu, et al., Structure, 13, 1665 (2005)].

Other improvements to the macromolecular beam lines and equipment include:

Automated Crystal Annealing – Using automated crystal annealing techniques has been shown to decrease mosaicity and improve diffraction for some macromolecular crystal systems. SSRL now offers two methods for users (on-site and remote) to anneal samples, implemented through the Blu-Ice software and GUI. During data collection, a cold nitrogen gas stream flows over the crystalline sample to maintain it at cryogenic temperature. The crystal is annealed by letting the sample warm up to near room temperature, and then quickly cooling the sample back down to ~100 K. This may be initiated from the Blu-Ice GUI by either of two methods: stream blocking or flow control. The annealing time for both methods is set by the user and requires confirmation to prevent accidental annealing. The stream blocking method uses a thin paddle to physically block the nitrogen gas stream near the sample position. This device may be controlled through the Blu-Ice GUI or manually by pushing a button located on top of the unit. The paddle position is encoded so that the control system may check if the paddle is fully retracted out of the gas stream. The device is also spring-loaded so that the paddle will retract if power to the device is lost. The main advantage of the stream blocking method is that the nitrogen stream is quickly blocked and unblocked, rapidly warming and cooling the sample. The second method for annealing, flow control, uses the nitrogen flow control settings of the beam line Oxford Cryojet to turn off the nitrogen stream from within the Cryojet dewar. Because there is a long transfer line between the Cryojet dewar and the sample position, the flow rate of the nitrogen stream at the sample position is changed more gradually than with the stream blocking method. The main advantage of the flow control method is that only the cold nitrogen stream is stopped and a warm nitrogen stream remains protecting the sample from icing.

A Universal Sample Container for Robots – The availability of robotic sample mounting systems continues to increase at synchrotron sources. Most of these are compatible with standard Hampton-style magnetic cryo-pins. However, many types of containers are used for sample pin transportation and storage. Currently at synchrotron beam lines in the US there are three main sample storage containers used: the SSRL cassette, the ALS puck, and the Molecular Structure Corporation (MSC) magazine. In an effort to minimize compatibility problems for users, a collaboration was initiated by SSRL with the ALS, the APS SBC-CAT and the industrial company, MSC to develop a “universal puck”. The universal container resembles a standard ALS puck. The key differences between the universal container and the original ALS puck are that the universal container incorporates features of the SSRL cassette and provides the same clearance around the sides and bottom of the sample pin as an MSC magazine. Both the universal container enclosure and the base contain magnets and the assembly will be held together by magnetic force. All compatibility issues have been addressed, and the approach is believed to provide complete transferability between all three systems.

The enclosure piece of the universal container will be used inside the SSRL SAM dispensing dewar. To adapt the enclosure piece to the standard SAM setup an adaptor cassette has been developed. Four universal containers may be inserted into one adaptor cassette. The adaptor is then inserted inside the SSRL robot dispensing dewar using the cassette transport handle. Within the SSRL dispensing dewar the three cassette locations will each be capable of interchangeably holding an SSRL cassette or an adaptor cassette containing four universal containers. Once the development is complete, SSRL users will have the option to use SSRL cassettes (with a 288 total capacity) or universal containers (with 192 samples) and the SAM system will be programmed to sort samples between these two options.

Beam Line Simulator – Funding has been secured from a combination of DOE BER, NIGMS and NIH NCRR to build a macromolecular crystallography off-line facility for crystal screening and beam line instrumentation and software development. This facility will contain the electronics, computing, software, and hardware required to mimic a typical SSRL crystallography beam line (albeit with much lower flux). A Rigaku MicroMax X-ray generator source that was procured by the Joint Center for Structural Genomics (JCSG) group with funding from NIGMS will be installed on this facility. JCSG will also fund the robotic system and both JCSG and SMB Core staff will participate in the construction of the beam line simulator. A BER-funded high-precision computer-controlled table is in production. The control electronics and the final beam definition system have been acquired with funding from NIH NCRR, while NIGMS funds are used to procure a kappa goniometer, Cryo-Jet Cryo-cooler, detector positioning system and associated electronics. BER funding is also providing the enclosure and the infrastructure. Benefits of this off-line resource include: 1) an increase in the rate of new developments without using precious synchrotron beam time, 2) the availability of an additional screening facility, 3) provision for an X-ray source for screening when SPEAR3 is down and 4) use as a training location where new staff and users can receive hands-on instruction.

X-ray Absorption Spectroscopy

BL9-3, dedicated to general user biological XAS, accepts 2 mrad of the wiggler fan as a side station on the 16-pole 2-T hybrid wiggler BL9. BL9-3 provides extremely high intensity over a broad X-ray energy range, with focused beam from ~4 keV to ~35 keV. The LN2-cooled monochromator has two sets of Si(220) crystal pairs that can be brought in and out of the beam without breaking vacuum, providing a choice of two different azimuthal orientations of the 220 planes with different glitch patterns, allowing a choice for a given element/energy range. Its M1 optic (after the monochromator) is configured such that the table does not move vertically during scanning, providing excellent stability and thus data quality. With SPEAR3, the focal spot is ~0.4 x 3 mm FWHM, producing a measured flux of ~2.2x10¹²/100 mA at 9000 eV with 1x4 mm apertures. A factor of five will be gained at 500 mA. With careful adjustment of the collimating M0 mirror, the beam line produces an energy resolution close to the theoretical limit. It is a superb BioXAS station and has enabled studies at µM concentration levels, bringing the experimental capabilities closer to physiological metal level in biological systems.

The upgrade for SPEAR3 500-mA operation was described above. The major impact of this upgrade has been to improve the beam line shielding and install beam line masks, slits, filters, and windows for the higher heat load. A significant additional improvement resulting from this upgrade is that the new pivot mask and beam line breakout in the SPEAR3 shield wall have made it possible to operate the BL9-3 M0 mirror at lower incidence angle (thus higher energy cutoff). The operating range was during the FY2004 and FY2005 runs limited to ~5-23 keV. The new operating range at lower mirror angle will allow focused operations >30 keV. In the future, if user demand so requires and with additional shielding, fully unfocused operations will be possible with the upper energy cutoff determined by the Si(220) monochromator crystal limit of ~45 keV. A second improvement is that the liquid nitrogen feed lines for cooling the monochromator crystals were re-designed to allow remote operation by improving the routing of the lines. The previous lines required entry into the optics hutch (closing all three BL9 branch lines) to route the lines manually to avoid catching on other BL components and causing damage. This change will provide a significant improvement in efficiency for user-requested monochromator crystal changes.

In the 2004 submission, it was proposed and strongly endorsed that the current capabilities for measurements in the 2-5 keV range be moved from BL6-2, and that BL3-3 (a bending magnet beam line) be rebuilt for this purpose. It has more recently been realized that rather than upgrade the BL3-3 soft X-ray station in its current location, this branch line could be incorporated as a branch line on a new bending magnet line, BL14. This would allow for a more optimal optics layout and hutch setup, as the location provides for a “green-field” approach, at what is estimated to be the same cost. The optical concept for the station envisions a collimating mirror followed by a double crystal monochromator and a refocusing mirror. This beam line will be located in Building 130 between the new location of BL4 and BL13. Detailed design for this new beam line began FY2006.

Other development projects in FY2006 include:

Installation and Commissioning of a Hard X-ray Fluorescence MicroXAS Imaging System – A fluorescence microXAS imaging system, based on Kirkpatrick-Baez (KB) mirror optics, was made available to users on BL6-2 during FY2006. The KB optics were purchased from Xradia Inc. The KB optics is mounted on an optical rail inside a helium box. This He path reduces air absorption and scattering of the primary X-ray beam, providing higher flux and less signal background. The setup contains a high-precision slit aperture mounted downstream of the first, vertically focusing KB mirror, which enables varying the intercepted beam size of the optics. This allows trading focused flux for spot size by probing different areas and locations of the mirror surface. The commissioning began with the characterization of the X-ray beam at the location of the virtual source slit downstream of the KB optics. It was found that the beam line focus at this position was very close to the theoretical beam size obtained by ray tracing calculations. Depending on the size of the virtual source, a smallest focus size of about (2 (horz) x 3 (vert)) μm²with approximately 5x10⁸ photons/sec/100 mA was measured by scanning a 100 μm diameter tungsten wire through the X-ray focus. The largest focus size is about (15 (horz) x 11 (vert)) μm², which is limited by the size of the beam line focus at the virtual source position.

Samples are mounted on a high-resolution x,y,z,theta sample stage allowing for sub-micron step sizes using a goniometer head with various sample mounting options for different types of samples. The sample itself can be observed by a high resolution long distance optical microscope with manual adjustable zoom, which is attached to a video camera/video server. A pellicle coated with about 100 nm of aluminum allows observing the sample in beam direction as the beam is scanned across the sample surface. This optical microscope is attached to a CCD camera/video server read-out and will simultaneously enable to optically locate and monitor certain areas ofinterest of the sample, which can then be easily moved in and out of the focal position. Currently the entire emitted fluorescence spectrum is detected as a function of position on the sample by an existing single element Si(Li) detector. In addition, a high resolution X-ray camera was developed in FY2005. This camera uses a very thin Gadox (P43) scintillator. An optical lens creates a magnified image of the X-ray beam on the scintillator onto a video camera with a magnification of about 2.5:1. This camera is important for characterizing the shape of the X-ray beam at various locations in the X-ray hutch. This camera improves and facilitates the optical alignment of the entire beam line as well as of the fluorescence microXAS imaging instrument. The K-B optic was funded by DOE BES and additional equipment and partial staff effort is provided from the SMB XAS program.

Instrumentation Development path is enclosed in a helium atmosphere. The existing setup achieves this by using flexible tubing which connects to a Plexiglas slit box (which includes a fluorescent screen for beam alignment) and then to the sample box. The setup requires manual adjustment of the slits and can only accommodate one sample at a time. Since at these energies external calibrations are necessary, frequent changes are required between the sample(s) of interest and the reference calibrant. This requires that the user enters the experimental hutch, changes the sample, and then allows the helium atmosphere to re-equilibrate. Although the setup is functional, significant improvements have been defined and a design is underway that will enable higher quality data, enhanced ease of use and more efficient beam time utilization.

Motorized slits in a KF-40 flanged vacuum- and helium-compatible configuration have been procured as a replacement for the manual slit system. The slits will be coupled to the beam line exit port and the sample box space using vacuum-compatible bellows, which will reduce potential helium leakage. Down-stream of the slits, a 6-way cross will be used to house a motorized linear motion feed-through, which will have both a fluorescent screen for sample alignment and reference calibrants for external calibration. A view port on one side of the cross will allow for viewing the beam size and position using an Applied Scientific Instruments LC150 camera, with a display at the work area. The opposite port will house a photodiode for fluorescent measurements from the reference calibrant. The design of the sample box space is still in progress, and will include both a shutter system (to minimize beam induce radiation damage) and a sample cooling system. During this year sample cooling options have been explored and tested, including a contact-cooled sample block (to -90º C), a Peltier-cooled setup, and a LHe flow cryostat with internal photo-diode detectors. In addition, improvements are being made to the existing detector electronics. This modified setup will be used on BL6-2, and will eventually be moved to BL14, when this beam line has been built (see below).

XAS Instrument Control System Software and Computing Developments – The SSRL developedInstrument Control System (ICS) forms the primary interface between control hardware and data collection software. The ICS software provides a consistent, convenient yet flexible interface to a large variety of different types of beam line hardware. These include, but are not limited to motors (D.C. and stepper), real-time clocks, counters, analog-to-digital converters and digital-to-analog converters. ICS supports devices controlled using CAMAC, VXI/VME, PXI, network, and RS232 based technologies. At present ~70 different devices are supported. The ICS software also includes a comprehensive set of subroutines for client programs that provide easy-to-use access to all devices in both a synchronous and asynchronous manner. During FY2005 several improvements, modifications and new instrument interfaces were added to the ICS software. It should be noted that all such changes have been made in such a way so that they can be easily ported to the new ICS software (see below) with a minimum effort. New device interfaces added during FY2005 include: Lakeshore Temperature Controller, V535 VXI Stepper Motor Controller, Proteus XES Stepper Motor Controller, Newport XPS D.C. Servo Controller, Ortec Analog-to-Digital Converters, M550 CAMAC LVDT Reader, and a Vortex & XIA DXP Detector Electronics Interface. During the year, development was started on ICS interfaces for both the Radiant Vortex-EX and XIA DXP detector electronics. The Vortex software was used very successfully on BL9-3, both by SSRL staff and visiting scientists. It is anticipated that development of both the interfaces will continue in FY2006 and beyond.

Although the current production version of ICS (V.1.22) provides a robust, reliable and fully featured software interface, it can only be used on Alpha CPU based computers running the OpenVMS operating system. Unfortunately, Alpha CPU based technology is in the process of being phased out. A transition to different platforms is being conducted in three main areas: (1) the development of an Operating System Independent version of the ICS software; (2) the migration of SSRL Data Acquisition Software to the new Operating System Independent model; and (3) new computer systems to support legacy software during the transition away from Alpha based CPU computers. Significant progress was achieved in all three areas, and the new ICS software was commissioned on BL7-3 in the spring of 2006. s for XAS in the 2-5 keV Energy Region – An improved setup for XAS measurements in the 2-5 keV range is currently in development, in preparation for the new capabilities to be provided by a new bending magnet beam line, as proposed in the program proposal. In this energy range, X-rays have a very short path length in air and thus the entire beam.

Also, significant progress was made in the design and development of a dedicated beam line network. Specifically a dedicated firewall machine was purchased, a Fortinet Fortigate 1000 (with a combination of DOE-BER and SMB DOE-BES funds). This firewall machine will be installedas a “fail-over” partner to an existing Fortigate 1000 firewall that already protects the SPEAR3 and Injector control systems. In addition, progress was made on the design of the Virtual Local Area Networks, or VLANs, that the firewall will provide. The design of the network is being undertaken with the assistance of the SLAC computer security team. It is anticipated that BL7-3 will be the first beam line to take full advantage of the new network architecture.

Small Angle X-ray Scattering/Diffraction

SAXS/D Beam Line 4-2 Operations – The SSRL small-angle X-ray scattering/diffraction facility on BL4-2 is dedicated to small angle X-ray scattering studies on biological systems. During the FY2006 run, the excellent beam characteristics created by the combination of the new low-emittance storage ring and the 20-pole wiggler, provided users with enhanced capabilities. Especially notable was the high stability of the photon beam as a direct result of the top-up injection scheme, which typically took 2-3 minutes three times a day, and helped stabilize the overall performance of the optics components. The stored electron ring current changed less than 20% between fills at 60+ hr lifetimes and the practically constant thermal load on the optics components resulted in the high beam stability. A number of user/staff research projects benefited strongly from these beam characteristics. It is also important to point out that several new exciting scientific projects were also initiated during FY2005, suggesting further growth in beam time demand and continuing need for advanced instrumentation for conducting challenging experiments.

SAXS/D Hardware Upgrades – In order to take advantage of the high-brightness SPEAR3 beam, a new instrument was built in FY2004. The main goals of this development were to 1) extend the range of instrument to both large characteristic length as well as to higher structural resolution, and 2) allow multiple instrument configurations during short beam time by automation. The new instrument incorporates the ability for automatically changing the sample-to-detector distance and a pair of built-in collimator/analyzer crystals in Bonse-Hart geometry for ultra-small angle X-ray scattering (USAXS) studies. The former feature allows users to select a desired sample-to-detector distance, thereby angular range, among five pre-designed distances, 0.5, 0.9, 1.4, 2.0, and 2.5 m. The Bragg spacings, d=1/(2sinθ/λ), in Å covered by these distance ranges are: 4.6-270, 8.7-520, 13-790, 18-1100, 23-1400, respectively, assuming the use of a 9 keV X-ray beam and a 5-mm-diameter beam stop.

Development projects include:

Camera Improvements – For the new camera, developments focused on automated camera distance change and related improvements: (1) Several hardware improvements were made to facilitate distance changes. Three stages on a common optical rail, individually supporting the most upstream flange of the scattering path, the sample stage and the slit assembly, were linked together in such a way that a single motorized motion moves all at the same time, eliminating manual translation of the sample stage. (2) A new collimator assembly was built, consisting of two in-vacuum slits built by Advanced Design Consulting, two flexible bellows and vertical and horizontal translation stages. The distance between the two slits is adjustable between ~0.5 to 1.2 m, so that the up-stream slit can be used as a beam defining slit for shorter sample-to-detector distances. (3) The vertical translation stage for the sample position was replaced with one that provides a longer range. (4) Smaller hardware improvements for the camera setup include the design of a new static measurement cell holder, which features dry nitrogen paths around each sample cell slot to prevent moisture condensation so that measurements can be performed at temperatures significantly lower than room temperature, e.g., 4°C. The ability to simultaneously achieve quasi-anaerobic conditions by nitrogen purging can be useful for those samples that are sealed in the standard sample cell, but normally require handling in a glove box filled with nitrogen gas. (5) The sample alignment video microscope was equipped with miniature vertical and horizontal translation stages and the cross hair can now be aligned with the direct beam position in the Blu-Ice software sample monitor window so that the user simply clicks on the exact location of the specimen to bring it into the beam (cross hair) automatically. (6) A split ion chamber for monitoring vertical beam position drift was built and installed. It is mounted on a precision motorized slide for vertical translation so that one quick vertical scan provides exact beam position at the most upstream location within the BL4-2 experimental hutch. All split ion chamber signals (top and bottom plates, difference and sum of them) can be monitored in Blu-Ice to keep track of direct beam position in case of unexpected beam positional shifts. The integrated ion chamber signal will be used as an incident beam intensity monitor when the new in-vacuum solution cell (see below) is used since the standard ion chamber, which records incident beam intensity immediately up-stream of the sample position, has to be removed when the in-vacuum cell is used.

Design and Construction of a New In-vacuum Solution Sample Cell – The first-generation in-vacuum cell performed well, but required substantial effort to align in the X-ray beam due to a few geometrical constraints. A new in-vacuum solution sample cell has been designed to eliminate these shortcomings while also providing additional new features. An X-ray capillary is mounted on a thermostated jacket, which is attached to a motorized vertical slide for alignment as well as for inserting and retracting the capillary cell in and out of the beam. The new in-vacuum cell will be permanently integrated with the pin-hole camera setup at the most upstream section of the scattering path, eliminating the need for installation and alignment in the beam each time it is needed. The small dimension of the enclosure (~2” in the beam direction) makes it possible to obtain higher angle scattering data by the use of a standard flat window solution cell holder immediately upstream of the in-vacuum cell. This way it is possible to quickly switch between the in-vacuum capillary cell and other sample holders. The black anodized enclosure of the new in-vacuum cell has six standard optic fiber receptacles for simultaneous light scattering and absorption measurements as well as illumination. A large glass window on the side is used for illumination and sample monitoring via a video camera, and a specifically designed port on the opposite side is used for exchanging the X-ray capillaries. The new in-vacuum cell is currently in assembly and will be commissioned in actual solution scattering experiments in FY2006. A Hamilton sample dispenser and a remote-controlled valve have been recently acquired. These are programmable devices either by the built-in controller of the dispenser or any computer via RS232 protocol. In FY2006 we will combine these devices with the new in-vacuum cell, a development aimed at high-throughput solution X-ray scattering studies. We intend to obtain a programmable sample changer, which would allow users to select one sample solution among many in the standard 96-well plate and send it to the capillary cell. The dispenser would enable dilution series to be measured.

Software Developments and Improvements of the Computational Environment – Running SAXS/D experiments on BL4-2 previously required the use of separate beam line control and data acquisition software packages, which lacked communication between them. In order to solve this problem, we installed and customized the Distributed Control System (DCS) and the accompanying graphical user interface (GUI) Blu-Ice (developed for macromolecular crystallography by the SSRL SMB MC group) for small-angle X-ray scattering data collection at BL4-2 in FY2004. The DCS/Blu-Ice package brings a number of notable benefits including: (1) proven stability, (2) in-house origin and the large knowledge base at SSRL within the SMB group, (3) IP based client/server model that allows integration of the integrated control system (ICS), which is SSRL’s other beam line control software, with data acquisition, (4) remote viewing and/or control of the experiment, (5) script based language and object oriented architecture that permit customization of existing features and addition of new functionalities and capabilities, and (6) X-windows based graphical user interface package (i.e. Blu-Ice) that can be run on multiple computer operation systems such as Microsoft Windows, Linux, Unix, and OS X (Macintosh).

Additional customizations and numerous improvements have been made to DCS and Blu-Ice to enable beam line staff members to condition the beam line and users to acquire data more efficiently. These improvements include: (1) automation of slit optimization, (2) automation of sample position search, (3) automation of camera length change, (4) automation of sample positioning, (5) new capabilities for multiple sample measurement, (6) new features to conditionally allow users to modify data acquisition conditions. Modifications were also made to a section of DCS so that data acquisition conditions can be recorded for later use in data analyses. Both the incident and transmitted beam intensities, integrated over the actual CCD exposure time, are now recorded synchronously, and they are written in the header portion of each 16-bit TIFF image files generated by the BL4-2 MarCCD165 detector. The same information and other experimental parameters are stored in a separate text file for future use.

Frequent sample-to-detector distance change is important for SAXS/D experiments to cover a wider range of scattering angles during a single slot of beam time. Additions to the Blu-Ice software in FY2005 have implemented automatic checking of vacuum levels and other sensors that are activated during camera length changes. If the sensors detect a fault, the camera length change will not proceed until corrections are made. The sample scan GUI was completely reworked. The users can find samples from the sample view image, set data acquisition parameters for each sample (up to a total of 10 samples), and, with one click, start data acquisition on all the samples. For each sample, the user can find the sample position in three alternative ways. Once the sample position is determined, the position parameters are automatically entered into a scan parameter table, and the user, for each sample, can set exposure time, number of exposures, and time intervals between each exposure, allowing for dynamic studies. This new GUI has made data acquisition using the MarCCD165 detector much more intuitive and effective at the same time, and has made user experiments more productive.

Bio-SAXS/D Linux File Server Improvements – During the FY2005 summer shutdown, we acquired a new and faster Linux PC as well as a larger capacity RAID disk array (860 GBytes), both purchased using DOE-BER funds, to completely replace the original BL4-2 computer in order to be ready for increased overall data throughput at BL4-2. The new file server also runs a Samba server program so that any SSRL Windows XP PC can read and write data files on the Linux computer directly over the network.

Molecular Environmental and Interface Science

Synchrotron radiation (SR)-based techniques provide unique capabilities relevant to environmental remediation science, and have emerged as major science and technology R&D tools in this field. The high intensity of SR sources coupled with X-ray photon-in/photon-out detection allows noninvasive in situ analysis of dilute, hydrated, and radioactive samples. SR techniques can be used to characterize the structural chemistry of non-crystalline solids, nanoparticles, environmental interfaces, bacteriogenic minerals, complex organic materials, and of metals sequestered within bacteria, plants, and dissolved in solution. Further, because of their high degree of collimation, SR X-rays can be focused to beams of micron dimension, allowing spatially resolved characterization of chemical species in microstructured samples, chemical microgradients, and microenvironments that often fundamentally control the behavior and release of contaminants in the environment, in waste forms, and in contaminated vessels.

BER Funded Staffing and New Opportunities for MEIS R&D

The goal of this program is to provide user support for BER ERSD-funded environmental remediation scientists and their collaborators at SSRL. This is accomplished through an integrated approach involving direct hands-on support, technique development, education and outreach efforts, and instrument development. Software support for instrumentation control and data acquisition is provided through a parallel effort for biological applications in the SSRL SMB XAS program, with BER-funded personnel.

User Support and Instrument Development – The key element of this user-support program is an ERSD-funded scientific staff member (Dr. Samuel Webb), who provides advice to users regarding experiment planning, hands-on training assistance at beam stations, and consultation regarding data analysis. Major techniques supported include X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), microbeam X-ray fluorescence chemical imaging (μ-XRF), and microbeamXAS/XRD. Another major element of the program is the development of innovative techniques for ERSD research. Presently, a microbeam spectroscopy/diffraction facility is being develop, optimized for experiments on U, Np, Pu, Tc, and other radionuclides that require high-energy X-rays, as well as for nonradioactive contaminants such as Cr, As, Se, Cd, Hg, and Pb. The microbeam facility is comprised of a Kirkpatrick-Baez (K-B) mirror pair, which focuses incident SR X-rays to a 2 micron spot, and various detectors, and includes sample positioners, video cameras, slits and ion chambers, all of which are mounted on an ambulatory optical bench. This facility will significantly enhance access to SR microbeam techniques for ERSD researchers, particularly to those national laboratory and academic programs located in the Western US.

Environmental Remediation Science Support Program at SSRL

BER-ERSD projects were conducted on beam stations BL11-2 and BL2-3 (XAS) and BL2-1 and BL11-3 (SR-XRD). A dedicated project scientific staff member, Dr. Samuel Webb, was hired to provide user support for BER-funded researchers, to help commission the microbeam system, and to implement the user support program at the microbeam system. Dr. Webb has 8 years of experience in the field of XAS-based environmental sciences, including three years as a postdoc at SSRL in Dr. Bargar’s group, a demonstrated mastery of the experimental techniques required for the position, and a demonstrated strength in user support.

User Education and Outreach –A web site was created for the SSRL-based environmental remediation science community (http://www-ssrl.slac.stanford.edu/mes/remedi/index.html) to provide key information and links to resources for these users, including contact information, information on submitting proposals and obtaining beam time, beam station resources at SSRL, science highlights, and a primer on the application of synchrotron techniques to environmental remediation science. A workshop entitled, “Applications of Synchrotron X-ray Scattering Techniques in Materials and Environmental Sciences,” was held on May 16 and 17, 2006.

Instrument Development – Activities centered on the design, implementation, and initial testing of an X-ray microprobe for μ-XAS, μ-XRD, and μ-XRF measurements. The microprobe is optimized for experiments on radionuclides of interest to BER researchers including U, Np, Pu, Am, and Tc, and it also provides experimental capability for a range of heavy metals, including Cr, As, Pb, and Sr. Initial hardware design, fixture fabrication, and assembly were completed in FY2005 and user operation began in FY2006. A high-resolution fast X-Y-Z scanning stage (required for μ-XRF imaging) and state-of-the-art digital spectroscopy amplifiers (DXP, Inc., required for both μ-XRF and μ-XAS capabilities) were procured and will be implemented during the current FY. The microprobe produces a focused beam of 2 µm diameter, which is close to the theoretical limit, and a flux of 3x10⁷ photons/s at 20 keV (100 mA current). A great deal of attention was focused on the mechanical stability of the system. The effective beam spot drift rateswere reduced to 6.3 nm/h (horizontal) and 125 nm/h (vertical), well below the measured spot size of the beam. μ-EXAFS measurements on a 10 µm-diameter Mo wire (20 keV) show a high degree of reproducibility and low noise, suggesting that the mechanical stability of the system is adequate for planned measurements.

Capital Equipment – An X-ray area detector for μ-XRD measurements has been ordered and is expected to be commissioned in FY2007.

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13. Progress in SSRL Operations by Piero Pianetta Appendix B Self-Evaluation FY2006