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

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