Michael Bedzyk, PhD - US grants

This project is in the area of the Materials Chemistry Initiative. The investigators will use an x-ray standing wave in the study of electrochemical interfaces. They will employ this technique in the study of ionic gradients at the electrode/solution interface including potential dependence studies. In addition, they will study the structure of electrodeposited layers; both metallic and polymeric by combining the x-ray standing wave technique with surface EXAFS spectroscopy.

Cohen 9304725 Northwestern University, E.I. Du Pont de Nemours and Dow Chemical have formed a Collaborative Access Team, known as DUNU, and are proposing to instrument and operate a sector at the Advanced Photon Source. A funding schedule, with costs to be shared equally by both parties has been agreed upon, covering both a five year construction phase, as well as a ten year minimum operations phase. Currently DUNU employs three full-time senior scientists and engineers headquartered at Northwestern (a nationwide search is on for a fourth senior scientist), with extensive experience in beam line construction and operations both at the Cornell High Energy Synchrotron Source (CHESS) and the National Synchrotron Light Source (NSLS Brookhaven), plus management of one of the largest x-ray diffraction laboratories in the country. The purpose of this Collaborative Access Team is to carry out research on the structure of advanced materials. Our understanding of the structure (atomic to micron level) is a crucial prerequisite to the development of new materials with enhanced properties. Synchrotron radiation has become an essential tool in every aspect of structural analysis and has revolutionized many subfields of science and engineering. Many members of our CAT have been very active users of our national facilities. The vastly increased capabilities of "third generation" machines, such as the APS and similar storage rings in Europe and Japan, promise a second revolution. A broad research program has been formulated, involving currently more than 30 principal investigators from Du Pont and Northwestern. At least fifty other scientists and engineers, students and post-doctoral fellows will ultimately be involved. Although many fields of materials science and engineering are represented in this research program, the research to be supported by the instrumentation in this project has a particular focus: It deals with materials, whose intermediate scale structure (nanometer to micrometer) has a profound influence on their properties. Such materials are of immense technological importance and include precipitation-hardening alloys, ceramics, polymers, cement and composite materials. Many elements of this research program are totally beyond current capabilities at available x-ray sources (conventional and synchrotron). The instrumentation that we will develop depends heavily (or crucially) on the characteristics of the APS. It is grouped in two experimental stations, both utilizing undulator radiation: 1. General purpose scattering instrument, suitable for all types of diffraction experiments (amorphous, powder and single crystal work, surface and interface diffraction, standing waves etc.). Collimated beams (0.1-1 mm) and microbeams will be available for diffraction and microprobe work. The microbeam capability is of particular interest in the context of this proposal. 2. Small Angle X-ray Scattering microtomography instrument equipped with 2-D position sensitive detector and environmental chamber with multiple attachments (low and high temperature, UHV, sample changer). This may be the first x-ray instrument with sufficient imaging and scattering resolution to allow the two techniques to overlap in studying micron- sized structural features. We believe that DUNU possesses all the key ingredients of a productive and strong Collaborative Access Team. Northwestern University anchors an interactive research community through its interdisciplinary research centers (many of which, such as the Materials Research Center and the Center for Advanced CementBased Materials are supported by the National Science Foundation) and individual faculty research. E.I. Du Pont de Nemours & Co. brings its immense capabilities in scientific research and development. It is a world leader in process chemistry and engineering and t he manufacturing of high technology products. It is the intent of this collaboration not only to share a station at this major facility, but to develop interactive, collaborative research between the two institutions (we have installed a videoconferencing link to aid this interaction).

9705473 Halperin The Ambitious goal of this program is to understanding the mechanism for superconductivity in heavy fermion compounds. The first step is to identify the symmetry of the order parameter and we focus first on Upt3, which is arguably the prototype unconventional heavy fermion superconductor. In order to determine the pairing state in Upt3 we propose to make single crystals with a range of impurities and measure the impurity dependence of the physical properties, including the thermal conductivity, specific heat and the phase diagram. In order to identify the spin structure of the pairing state, we will measure NMR knight shifts and the upper critical fields and compare with calculations that are sensitive to surface scattering and strong spin-orbit interactions. We will perform measurements of magnetization and magnetoresistance and, with our collaborators at other institutions, we will investigate further the magnetic phase diagram using neutron and magnetic X-ray scattering. We will compare these results with calculations of impurity and defect scattering on the physical properties for the principal candidate ground states. The low-temperature transport coefficients, eg. thermal conductivity, are important because they reflect the low enrgy excitations associated with the nodes of the gap. We believe that transport measurements on high quality single crystals, such as we are able to grow, will be decisive in resolving the orbital symmetry of the pairing state in heavy fermion superconductors, e.g. Upt3. %%% It is proposed to solve key problems in heavy fermion superconductivity with initial focus on the important compound Upt3. The results of this research will have impact on understanding of superconductivity in other correlated fermion superconductors, notably high temperature superconductors. That superconductivity in Up t3 is unconventional, is evident in its complex phase diagram and the power laws that are exhibited in the temperature dependence of thermodynamic and transport properties, particularly at low temperatures. Yet it is not yet known which of a number of likely possible candidates for superconducting pairing state is the appropriate model for Upt3. Separating intrinsic and extrinsic behavior is the key to solving this problem and is a challenge for experimental and theoretical condensed matter physics in concert with a materials program. Growing and characterizing the purest, most defect-free crystals possible is essential to the success of research in this field and we have developed facilities for this purpose. Interest has been expressed from an international group of scientists to collaborate with us, working with the materials produced in these facilities. The proposed work draws together four principal investigators, Professors Bedzyk, Halperin, Sauls and Seidman from two departments at Northwestern University: Materials Science and Engineering and Physics and Astronomy. These researchers have expertise in materials science, crystal growth and characterization, experimental low temperature physics, and theoretical physics of correlated electron systems and superconductivity. The expertise of the members of this group is at the forefront of their respective disciplines. ***

This project explores several key issues related to growth of group IV epitaxial materials by surfactant mediated processes. In situ studies will center on adsorption of elements from groups IV-VI on the (001) surface of Si and Ge single crystals to explore issues of general importance for heteroepitaxy. The approach is to first determine the structure of equilibrium and metastable surface phases of relevant monoelemental adsorbates. Next, adsorption on pseudomorphic layers will be studied to gain an understanding of factors such as lattice strain, steric effects, and relative chemical bond strength that determine the structure of a particular surface heterophase. The modification of the kinetics of surface processes by surfactants will then be explored, and exploited to facilitate growth of metastable epitaxial alloy structures. The structural perfection and lattice strain of the heteroepitaxial thin films produced will be examined as a sensitive monitor of the efficacy of the investigated growth technique. The study features x-ray standing waves, surface x-ray diffraction, and surface extended x-ray absorption fine structure spectroscopy, and focuses on the initial and secondary phases of strained-layer heteroepitaxy, as well as the kinetics of epitaxial growth. Specific aspects of heteroepitaxial growth to be examined are: (1)surface structure resulting from the deposition of a monolayer (or less) of a heterolayer, (2)surface structure of growth-related adsorbates (e.g., surfactants or dopants) on the surface of the substrate, (3)structure found on the surface of a thin strained heterolayer, and (4)alteration of surface kinetics by the presence of a surfactant species.
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The project addresses basic research issues in a topical area of materials science having technological relevance. An important feature of the project is the integration of research and education through the training of students in a fundamentally and technologically significant area.
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