34ID Beamline Construction
I. K. Robinson,* C. A. Benson
National Science Foundation, DMR 97-24294; U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We are constructing an undulator beamline at the Advanced Photon Source, Argonne National Laboratory. It will provide an intense dedicated source of x-rays for coherent x-ray diffraction experiments. The design uses a grazing-incidence, horizontally deflecting mirror to separate our beam from a second one, used by scientists from Oak Ridge National Laboratory. The beamline will be used for investigation of the microstructure of polycrystalline materials and the physics of fluctuating condensed matter systems.

Atomic Correlation in Disordered Materials Observed Using Variable Coherence Transmission Electron Microscopy
J. M. Gibson,* P. M. Voyles in collaboration with M. M. J. Treacy, NEC Research Institute
National Science Foundation, DMR97-03906

Using a novel quantitative electron microscopy technique, fluctuation microscopy, we are studying the atomic correlations in amorphous and disordered materials. In particular, we are examining the structural instability of a Si(H) which has important applications in solar cells and thin-film transistors.

Atomic-Scale Studies of Superconductivity and Magnetism with the STM
A. Yazdani,* D. Hornbaker, M. Vershinin, S. Misra
National Science Foundation, CAREER; Campus Research Board

We use a unique low-temperature STM to study fundamental issues of superconductivity and magnetism on the nanometer length scale. Our experimental program is divided into two areas: the interplay between magnetism and superconductivity on the atomic scale and the magnetic properties of nanometer-scale structures fabricated on surfaces. The STM is used to perform atomic-scale imaging, manipulation, and the tunneling spectroscopy. The combination of theses techniques provides a unique opportunity to examine the spatial variation of superconductivity near magnetic structures and to examine the magnetic behavior of STM-fabricated structures consisting of only a few atoms.

Atomically Uniform Films
T.-C. Chiang,* T. Miller, D. Luh
Petroleum Research Fund, American Chemical Society

Recent advances in crystal growth have made it possible to prepare atomically uniform films of Ag on an iron whisker. Films prepared by thermal evaporation onto a low-temperature substrate followed by annealing are investigated by angle-resolved photoemission. Electrons in the film, confined by the substrate potential, form discrete quantum well states. These states show atomic layer resolution and can be used as spectroscopic fingerprints for layer thickness identification. Our goal is to understand the kinetics and atomic processes involved in the formation of such uniform films that have been thought to be impossible to prepare until now.

Atomistic Studies of Silicon Oxidation
J. M. Gibson,* X. Chen
Semiconductor Research Corp.

We use in situ transmission electron microscopy to examine the fundamental mechanisms of silicon oxidation. In particular, the origins of interfacial roughness, which has serious impact on semiconductor device reliability, are studied using electron diffusion and imaging.

Charge Transport across Superconductor/Semiconductor and Superconductor/Normal-Metal Interfaces
L. H. Greene,* A. C. Abeyta, I. V. Roshchin, M. Matney, T. Tanzer (Chemistry), X. L. Li (Chemistry), W. L. Feldman, in collaboration with the research groups of D. J. Van Harlingen, P. M. Goldbart, P. W. Bohn (Chemistry), and J. F. Klem (Sandia)
U.S. Department of Energy, DE-FG02-91ER45439 (In cooperation with the Materials Research Laboratory)

This research program is a coordinated experimental and theoretical study of the static and dynamic properties of hybrid superconductor-semiconductor structures. Electronic transport, superconductive tunneling, magnetization, and light-scattering measurements are conducted on planar, microfabricated structures of high-quality Nb and NbNx thin films grown directly on III-V semiconductor heterostructures. Details of the superconducting proximity effect, Andreev reflection, and tunneling are investigated. We have performed the first optical detection of the superconducting proximity effect: Raman spectroscopy is the optical probe of an InAs interface in good electrical contact with a superconductor.

Coherent X-Ray Diffraction
I. K. Robinson,* I. Vartaniants, G. Williams, M. Pfeifer
National Science Foundation, DMR 98-76610

In these experiments, we prepare beams of x-rays that are so narrow that they are coherent across their width. A diffraction measurement with such a beam is representative of the entire object under illumination. An image of the object can therefore be derived by inversion of its diffraction pattern, using computer algorithms. We are currently applying the techniques to study the structure of microscopic grains inside materials. Individual crystalline grains give diffraction patterns characteristic of their shape, which can be reconstructed. We expect to be able to image the spatial distribution of strain within grains.

Contribution of the University of Illinois to the Magnetic Random Access Memory Project
M. B. Salamon,* M. B. Weissman, E. Nowak, S.-H. Chun
U.S. Army/IBM 2040

We are subcontracted by IBM to explore the basic properties of magnetic multilayer structures that might be useful for magnetic random access memory (MRAM) modules. Specifically, we are studying the noise characteristics of trilayer junctions that exhibit spin-dependent tunneling and examining the distribution of magnetic material in the structures by means of neutron reflectometry. The latter work is being performed at the Missouri University Research Reactor. Future work will involve detailed examination of tunneling characteristics, perhaps using superconductors in place of the magnetic top layer.

Development of an Ultrahigh Resolution Photoemission System for Studies of Quantum Structures
T.-C. Chiang,* T. Miller, D. Luh
National Science Foundation, DMR-9975182

Recent advances in experimental capabilities at national synchrotron radiation facilities including the development of new monochromators and high-intensity undulator beamlines have enabled ultrahigh-resolution measurements, which will provide detailed information about quasi-particle interactions and electron correlation effects in solids. To take advantage of these developments, we are setting up an ultrahigh resolution angle-resolved photo-emission system that will match the performance enhancements of the light source. A two-dimensional detector will be employed that will allow multichannel energy- and angle-dependent data to be taken simultaneously.

Electron Optics Research on Scalpel
J. M. Gibson,* K. Xiu
Semiconductor Research Council, SRC 98-MJ-629

We will systematically examine alternative electron-optical solutions for projection electron lithography. This demands a large field of view with low aberrations and has not been the subject of extensive research even though it looks likely to be a commercially viable method for fabricating semiconductor chips in 2010.

Electronic Properties of Impurities, Surfaces, and Quantum Structures
T.-C. Chiang,* T. Miller, D. Luh
National Science Foundation, DMR-9975470

High-resolution angle-resolved photoemission is employed to investigate the basic electron properties of metal surfaces and films. Atomically uniform films are prepared, and the resulting quantum-well states can be understood in terms of Fabry-Perot modes in a solid-state electron interferometer. An interferometric analysis yields the band structure and quasi-particle lifetime broadening that are the most accurate to date. Effects of impurity and defect scattering, both at the surface and in the interior of a film, will be studied by doping during film growth. Temperature-dependent phonon scattering and electron-electron scattering effects will also be investigated.

Excitations in Solids by Inelastic X-Ray Scattering
R. O. Simmons,* D. A. Arms, E. Burkel,* C. Seyfert, H. Sinn (Univ. of Rostock)
U.S. Department of Energy, DE-FG02-96ER45439; German Federal Ministry of Research and Technology (In cooperation with the Materials Resarch Laboratory)

Excitations in condensed matter systems have characteristic properties that are summarized in the dynamic structure factor (S(Q,E) where Q and E are the momentum and energy transfers, respectively, in a scattering process. Solid helium-three, the most extreme quantum solid, cannot be studied by conventional neutron scattering because of its enormous neutron absorption. Enough x-ray flux is now available, however, from undulators at third generation synchrotron sources to measure S(Q,E) from weakly scattering systems such as helium. At the Grenoble ESRF, the lattice dynamics of this extraordinary crystal are studied at low temperature and high pressure and compared to the isotopic sibling helium-four.

Experimental Determination of the Pairing State of Unconventional Superconductors
D. J. Van Harlingen,* B. D. Yanoff, J. E. Sadleir
National Science Foundation, DMR 97-05695

Experiments are underway to determine the order parameter symmetry of exotic superconducting materials suspected to have unconventional pairing mechanisms, leading to anisotropic energy gap structure. These include heavy fermion, organic, and ruthenate superconductors. By measuring the magnetic response of Josephson junctions and dc SQUIDs fabricated between single crystals of the exotic superconductor and conventional superconducting thin films, we can determine the phase anisotropy of the order parameter, distinguishing proposed anisotropic pairing states. Low-temperature penetration depth measurements using a resonant oscillator technique yield complementary information on the magnitude of the order parameter.

Experimental Determination of the Pairing State of the High-Temperature Superconductors
D. J. Van Harlingen,* J. E. Hilliard
NSF Science and Technology Center for Superconductivity (In cooperation with the Materials Research Laboratory)

We are probing details of the order parameter in the high-temperature superconductors by Josephson tunneling and SQUID interferometry measurements. Using edge junctions fabricated on cuprate thin films to define the tunneling direction, we can measure both the magnitude and relative phase of the order parameter in arbitrary directions. In particular, we are studying SQUIDs with junctions on the (100) and (110) faces of YBCO to test for surface-induced subdominant complex order parameters with broken time-reversal symmetry. We are also studying the effect of the loop inductance on the spontaneously generated magnetic flux in corner SQUIDs incorporating a d-wave superconductor.

Growth and Properties of Single-Crystal Films
C. P. Flynn,* M. Ondrejcek, C. Durfee
U.S. Department of Energy, LEEM, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The growth processes of single-crystal films prepared by MBE are investigated with a view to applications in rare earth magnetism, surface science, atomic mobility in materials, and the evolution of materials in radiation fields. A specific synthesis route starting with commercial sapphire buffered by bcc refractory metals is being explored. This research uses a low-energy electron microscope (LEEM). It permits exploration of surface morphology and reconstruction during actual growth and under conditions of ultrahigh vacuum at temperatures up to 1400?C. Through diffraction and imaging, the evolution of surface morphology including reconstructions, surface steps, slip bands, threading dislocations, and bulk morphology including screw, edge, and interfacial dislocations and subboundaries can be examined in real time and during actual growth.

In Situ Studies of Materials Growth
J. M. Gibson,* J. C. Yang, M. Yeadon, W. Henstrom, M. Kleinschmidt
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

This is a study of surface and interface structure using quantitative transmission electron microscopy. TEM studies are made of surface reactions and in situ epitaxial growth using image formation using surface-related diffracted intensities. Quantitative atomic resolution microscopy is being applied to interface structure and chemistry.

In Situ X-Ray Diffraction Studies of Surfaces and Interfaces
I. K. Robinson*
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We are operating a surface x-ray diffraction station connected to the same vacuum system as a number of MBE growth chambers in the EpiCenter central facility. The equipment has full three-dimensional capabilities and is optimally designed for structure determination at surfaces and interfaces of samples transferred directly from one of the growth chambers. X-ray diffraction data allow precise atomic-level structural information to be obtained. The analysis is nondestructive, so growth experiments may be continued after analysis. We are currently using this for determining segregation profiles in semiconductor and metal heterostructures, such as Sn/Ge(100) and Ge/Si(100).

Magnetic Behavior of Oxides and Nanophase Materials
M. B. Salamon,* H. Yanagihara, S.-H. Chun, P. Lin, S. Baily
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Certain manganese oxides, when doped, exhibit remarkable changes in electrical resistance at the ferromagnetic transition temperature. These changes are sensitive to magnetic fields, causing colossal magnetoresistance. Recent work focuses on the ferromagnetic state and the nature of the transition to it. We have identified a new mechanism for the Hall effect arising from quantum interference unique to these oxides and have used the Hall effect to pinpoint the boundary between polaronic and metallic regimes. The conductivity, thermoelectric power, and possibly the Hall coefficient can be treated in the regime near Tc in terms of two-phase behavior involving polarons and baud electrons.

Magnetic Imaging of Vortices in High-Temperature Superconductor Films and Devices
D. J. Van Harlingen,* B. L. Plourde
NSF Science and Technology Center for Superconductivity (In cooperation with the Materials Research Laboratory)

We are using scanning SQUID microscopy (SSM) to study vortex dynamics and pinning in superconductor samples. The SSM provides good spatial resolution and unparalleled flux sensitivity for imaging magnetic field distributions, making it useful for imaging vortex distributions around defect structures in superconducting thin films and crystals. We have imaged flux distributions and studied vortex motion around surface steps in NbSe2 crystals and in patterned Nb, NbN, and MoGe thin films, which exhibit different pinning strengths. These results are being compared with transport measurements of the critical current and magnetization measurements of flux entry and exit from superconductor samples.

Magnetic Screening and Phase Fluctuations in Underdoped Cuprates
D. J. Van Harlingen,* K. D. Osborn, J. A. Bonetti
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We are exploring the normal regime and the transition to superconductivity in underdoped cuprate thin films. These materials exhibit anomalous properties, in particular an apparent energy gap in the single particle excitations above the superconducting transition temperature. This pseudogap behavior has been described in terms of phase fluctuations, preformed pairs, inhomogeneous stripe formation, and spin dynamics. We are testing these ideas using two-coil susceptibility measurements and nanoscale transport measurements to probe the magnetic screening and local conductivity of superconducting films near and above the transition.

Microscopic Processes in Irradiated Crystals
R. S. Averback,* C. P. Flynn,* R. Appleton
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Fundamental processes of irradiation-induced defects in crystalline solids. High-resolution analytical methods such as TEM, SIMS, RBS are used to explore the atomic processes at the size scale of the defect events. Thermal spike behavior, radiation induced diffusion, radiation sputtering, and sink behavior are being studied. Experimental efforts are complemented by molecular dynamic computer simulations.

NMR Studies of High-Temperature Superconductors
C. P. Slichter,* C. Milling, R. Stern, I. Haase
NSF Science and Technology Center for Superconductivity (In cooperation with the Materials Research Laboratory)

NMR has proved to be an important tool to study superconductivity. We are investigating the normal and superconducting states of high-temperature superconductors such as YBa2Cu3O7-d or La2-xSrxCuO4, to learn how to describe the normal state, what mechanism leads to superconductivity, and why the transition temperatures are so high. The resonances of 63,65Cu, 17O, 89Y, 135,137Ba permit NMR to probe specific atomic sites (e.g., Cu nuclei in the CuO2 planes).

Nanoscale Studies of Inhomogeneous Electronic Phases in Magnetic Oxides
A. Yazdani,* D. Hornbaker
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Imaging and spatially resolved spectroscopy with a scanning tunneling microscope is being used to examine the nature of electronic states in magnetic oxides. The complex electronic interactions in these materials give rise to a number of novel phenomena such as colossal magnetoresistance and electronic phase separation. The goal of this project is to directly map out the spatial variations of the electronic states in these compounds on the nanometer scales, as a function of temperature and an external magnetic field. Such information will provide key information on the nature of underlying electronic interactions in these materials.

Noise Investigations of Condensed Matter Systems
M. B. Weissman,* E. Kolla, R. D. Merrithew, F. M. Hess, A. Mills, L. Chao, A. Palanisami
National Science Foundation, DMR 99-81869

The conductivity and other properties of most condensed matter systems show slow fluctuations, resulting from transitions between multiple metastable states. Especially in small samples, this noise can reveal aspects of the physics of materials, especially disordered materials, which are hidden in measurements of average properties. We are using this noise to study colossal magnetoresistive effects, relaxor ferroelectrics, Barkhausen effects in magnets, and other problems in condensed matter.

Nonequilibrium Studies of Photoexcited Carriers and Phonons in Semiconductors and Superconductors
J. P. Wolfe,* J. Short, R. Vines, J. Jang
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Photoexcitation of a semiconductor produces mobile electrons and holes which bind into excitons at low temperatures. The thermalization and diffusion of excitons in several semiconductors are studied by picosecond spectroscopy and imaging in the MRL Laser Lab. Photoexcitation of a superconductor produces nonequilibrium phonons which are used to probe the superconducting state of Nb and Pb. Acoustic waves are used to probe the transmission of periodic structures.

Nuclear Magnetic Resonance in Solids
C. P. Slichter,* C. Milling, R. Stern, J. Haase
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We probe magnetic and electric fields at the atomic level by NMR to study many-body effects, phase transitions, magnetism, solids possessing unusual properties, and electronic and structural aspects of surface atoms and absorbed molecules (including catalysis). Examples: Solids (1) high-temperature superconductors, for which NMR provides detailed information about both the normal and superconducting states; (2) organic conductors which exhibit both superconductivity and antiferromagnetism; (3) charge density waves (NMR of NbSe3) including study of the motion under applied electric fields; (4) Surfaces (a) electronic properties of the surface layer of atoms of Pt particles, by 195Pt NMR; (b) bonding and structure of molecules (e.g., CO, C2H2) adsorbed on Pt, by 13C NMR.

Phase Transitions in High-Temperature Superconductors
M. B. Salamon,* I. Bonalde, T. Park, E. Chia
NSF Science and Technology Center for Superconductivity

Recently, we have detected indications that high-temperature superconductors can undergo a second phase transition at low temperatures, driven by coupling between the d-wave Cooper pairs and magnetic impurities. Our research focuses on efforts to study that transition further through superconducting penetration depth. The low-temperature penetration depth of organic and p-wave superconductors is also under investigation. Further studies of the d-wave nature of the superconducting state are underway, including predictions of a dependence of the heat capacity on the orientation of a magnetic field.

Properties of Crystalline Condensed Gases
R. O. Simmons,* D. A. Arms, R. Shah
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Atomic motions and mobilities in condensed matter influence many important properties of materials. Condensed atomic and molecular gases form excellent prototype systems, since they exhibit phenomena in an enhanced form and can be studied under extreme conditions of density, temperature, and quantum effects. Synchrotron x-ray and neutron scattering is used to measure phase transformations, dynamics, such as momentum distributions, in liquid and solid He, H2, Ne, Ar, selected mixtures, C2F6, etc. Direct comparisons are made with path-integral Monte Carlo and other calculations. Separately, x-ray diffraction and pressure techniques are applied to study the properties of crystalline defects and intrinsic properties in He isotope crystals.

Properties of Simple Liquids and Glasses
A. V. Granato,* A. B. Lebedev, C. Gordon, W. Bains
National Science Foundation, DMR 97-05750

Critical tests of the interstitialcy theory of condensed matter states are being made by measuring the temperature dependence of the elastic constants of crystals just below the melting temperature in the crystalline state and just above the glass temperature in the supercooled liquid state. Observation of predicted results would confirm a simple, quantitative, easily visualized model according to which simple liquids and amorphous materials are crystals containing a few percent of self-interstitials.

Quantum Circuits at High Frequencies
R. Giannetta,* I. Adesida (Elect. & Comput. Engr.), J. M. White, P. P. Phillips
Campus Critical Research Initiative

In high-mobility semiconductor nanostructures, novel forms of electrical transport such as quantized conductance and Coulomb blockade have been firmly established. New, nonclassical phenomena are also predicted to occur in the time domain. These include quantum inductance and photon-assisted tunneling. Research into this regime of very high-frequency response is the focus of this effort. Our experiments require a combination of semiconductor nanofabrication, low-temperature transport measurements, and modern electrooptic techniques in the terahertz domain.

Quantum Statistics of Excitons in Semiconductors
J. P. Wolfe,* K. O'Hara, J. Warren, J. Guillingsrud
National Science Foundation, DMR 88-22761

Excitons, or bound electron-hole pairs, are produced by optical excitation of semiconductors at low temperatures. The excitons in Cu2O behave as an ideal gas of particles within the crystal. The volume of this gas is determined by the excitonic lifetime (nanoseconds to microseconds) and diffusion rate. As the laser excitation level is increased, the excitonic gas displays quantum statistics. We are examining the kinetics and thermodynamics of this unique gas and the occurence of Bose-Einstein statistics. The techniques used include time- and space-resolved photoluminescence following laser pulses of nanosecond and picosecond duration.

RF and Microwave Electrodynamics in High-Temperature Superconductors
R. Giannetta*
NSF Science and Technology Center for Superconductivity (In cooperation with the Materials Research Laboratory)

A number of different experiments indicate that high-temperature superconductors have an unconventional, d-wave type of pairing symmetry. Our goal is to understand how the electromagnetic response of these materials is altered by d-wave pairing. High-sensitivity radio-frequency oscillator measurements are being used to look for changes in the superconducting penetration depth indicative of new pairing states. In addition, we are using penetration depth measurements to study a wide variety of new superconducting vortex phenomena. These include vortex lattice melting, Josephson vortex viscosity, and connections of vortex dynamics to the pairing symmetry.

Raman Scattering from High-Temperature Superconductors
M. V. Klein,* M. Ruebhausen
NSF Science and Technology Center for Superconductivity; Deutsche Forschungsgemeinschaft (In cooperation with the Materials Research Laboratory)

Raman scattering is used to study magnetic fluctuations (two-magnon spectra) and electronic excitations in the cuprate high-temperature superconductors. Of particular interest are the superconducting gap spectra and their connections with gap spectra obtained on the same materials by spectroscopies that add or subtract a single electron. Changes in the spectra are studied as a function of temperature, carrier concentration, magnetic field, and incident laser photon energy. These studies are designed to improve our knowledge of how antiferromagnetic fluctuations affect superconductivity and vice versa.

Search for Subdominant Order Parameter Phases in d-Wave Grain Boundary Junctions
D. J. Van Harlingen,* W. K. Neils
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

We are carrying out experiments designed to test for the onset of subdominant superconductor phases induced by perturbations in d-wave superconductors. Phases with complex order parameters are predicted to arise from the suppression of the d-wave order parameter at surfaces and near magnetic impurities by Andreev reflection. We are fabricating grain boundary junctions in pure and magnetically-doped cuprate films and measuring the critical current at low temperatures, searching for signatures of a phase transition to a secondary order parameter state. The critical current modulation with applied magnetic field is also being monitored to extract phase-sensitive information about the order parameter.

Semiconductor and Ceramic Surfaces and Interfaces
T.-C. Chiang,* T. Miller, D. Luh, T. Kidd, M. Holt, Z. Wu
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Photoemission and x-ray diffraction and scattering techniques are employed to determine the electronic properties and the atomic structure of surfaces and thin films. The behavior of crystal growth by molecular beam epitaxy, chemical vapor deposition, laser ablation, and magnetron sputtering is being investigated. Key issues of interest include the chemical reactions and atomic interactions during thermal and energetic beam deposition leading to the formation of surface reconstructions, nonequilibrium compositions, and metastable structures. Fundamental surface behavior such as charge density wave transitions and metal insulator transitions are also of interest, and these phenomena are probed by photoelectron holography and x-ray diffraction.

Solid-Liquid Interface Studies by X-Ray Diffraction
I. K. Robinson,* A. A. Gewirth
U.S. Department of Energy, DE-FG02-96ER45439, DE-AC02-76CH00016; National Synchrotron Light Source (In cooperation with the Materials Research Laboratory)

We have constructed a high-speed diffractometer, employing the 'kappa' geometry, for use on the X16C beamline at NSLS, Brookhaven National Laboratory. The beamline has a focused beam of 1010 photons per second in a submillimeter spot on the sample. There we use a teflon environmental cell with a thin mylar window to hold samples inside liquids under full electrochemical control. The thin-layer geometry allows the transmission of the x-ray beam to the sample and out again. As part of a campuswide effort, we are studying metal and ionic adsorbed species on copper surfaces, to try to gain an understanding of copper and aluminum corrosion.

Spectroscopic Studies of Low Carrier Density Magnetic Systems
S. L. Cooper,* C. Snow
National Science Foundation, DMR 97-00716

We are interested in a number of low carrier density magnetic systems with rich phase diagrams as a function of doping, including insulating, ferromagnetic metal, as well as antiferromagnetic ground states. The diverse phase diagrams of these materials derive largely from the competition between strong Coulomb correlations, electron-phonon coupling, and spin interactions. We are using various optical techniques, including reflectance and light-scattering spectroscopies, to characterize the excitation spectra of these materials, and to elucidate the mechanisms driving the different phase transitions.

Spectroscopic Studies of the Magnetic Oxides
S. L. Cooper,* S. Yoon, H.-L. Liu
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

The magnetic oxides exhibit a wide variety of exotic phenomena, including paramagnetic insulating-to-ferromagnetic metal transitions and 'colossal magnetoresistance' behavior at intermediate doping, as well as ordered charge and spin structures at high doping. We are attempting to elucidate the physics governing these interesting phase regions by using reflectance, Raman, and Brillouin scattering spectroscopies to study the interactions between the lattice, charge, and spin degrees of freedom in these materials.

Studies of Amorphous Materials with Electron Fluctuation Microscopy
J. M. Gibson,* P. M. Voyles
National Science Foundation, DMR 97-03906

Using statistical measurement of fluctuations in higher solution transmission electron microscopy of amorphous thin films, we are examining medium-range ordering.

Submicron Magnetic Imaging
D. J. Van Harlingen,* T. A. Crane
National Science Foundation, DMR 97-05695

We are developing new techniques for magnetic imaging based on extensions of the scanning SQUID microscope (SSM). This includes variations of the SSM to improve spatial resolution, to enable image acquisition at ultralow temperatures, and to move/position vortices in arrays and superconducting films. The highest priority is to achieve submicron spatial resolution necessary to study the structure and behavior of vortices in superconductor systems. For this, we are investigating the use of single Josephson junctions as detectors. Our ultimate goal is to carry out experiments to study phase coherence and vortex dynamics in unconventional superconductors and metal/insulator systems.

Superconducting Gap Excitations in High- and Low-Temperature Superconductors
M. V. Klein,* H.-L. Liu, I. Yang
National Science Foundation, DMR 9705131

We study the superconducting gap in the cuprate high-temperature superconductors obtained using Raman scattering with various directions of light propagation and polarization. In this way, we can excite an electron either parallel to a single CuO2 plane or between adjacent planes. We compare with other spectroscopic data to gain information about interactions in the superconducting state. We also study the superconducting gap in the borocarbide family of 'conventional' superconductors. We vary the temperature, apply magnetic fields, and study the effect of impurities on the Raman spectra. Comparisons are made with the BCS theory and with the cuprates.

Superconducting Vortex Dynamics
M. B. Weissman,* M. Rabin
NSF Science and Technology Center for Superconductivity

The pinning and depinning of magnetic vortices determines whether a superconductor remains superconducting in a magnetic field. Individual vortices usually do not pin well enough to maintain good superconductivity, but collective vortex pinning can be very strong. We are using new noise techniques to study how the collective effects develop by which vortices strongly pin.

Superconductive Tunneling Spectroscopy and Electronic Transport in Pure and Doped YBa2Cu3O7 Thin Films
L. H. Greene,* E. Badica
National Science Foundation, DMR 94-21957

Reliable film growth, electronic transport, magnetization measurements, and superconductive tunneling provide the foundation for our investigations of the electronic properties of high-temperature superconductors. Thin films of pure Ni- and Zn-doped YBa2Cu3O7 are grown by sputter deposition. These thin films are also grown in various crystallographic orientations, allowing charge transport measurements along different lattice directions in this highly anisotropic material. Information on the disorder and interface properties of this unconventional superconductor is being provided through these measurements. Furthermore, tunneling provides a powerful spectroscopy of the superconducting state, which will help elucidate the mechanism of high-temperature superconductivity.

Tunnel-Junction Fabrication Using Chemical Techniques
L. H. Greene,* P. Hentges, E. A. Pugel, in collaboration with W. Klemperer, Chemistry
NSF Science and Technology Center for Superconductivity

To date, the most reliable method of tunnel-junction fabrication on high-temperature superconductors has been by evaporation of Pb counter electrodes directly on the YBa2Cu3O7 surface. A chemical interaction between these materials causes a reproducible, insulating tunnel barrier, but also a ~30A thick damage layer. To avoid such surface degradation, we are investigating chemical modification of the YBCO interface for formation of the tunneling barrier material.

Tunneling Spectroscopy of High-Temperature and Other Unconventional Superconductors
L. H. Greene,* E. A. Pugel, H. Aubin
NSF Science and Technology Center for Superconductivity

We take advantage of the unconventional nature of high-temperature superconductors to probe details of the superconducting and normal-state properties. Tunneling spectroscopic studies of the surface-induced Andreev bound state (ABS) are performed. This (ABS) consists of quasi-electrons and quasi-holes that form near to the interface of an unconventional superconductor, such as the d-wave superconductor, YBa2Cu3O7. Investigations of this ABS as a function of several physical parameters, including temperature, applied magnetic field, crystallographic orientation and disorder reveal signatures of intriguing broken symmetries in these complex superconductors, including spontaneously broken time-reversal symmetry. Other single-crystal unconventional superconductors, and novel techniques for exploring the broken symmetries are also being explored.