Photoemission and scanning tunneling microscopy techniques are employed to determine the electronic properties and the atomic structure of surfaces and thin films. The behavior of crystal growth on surfaces by molecular beam epitaxy and chemical vapor deposition is also investigated. Key issues of interest include the chemical reactions and atomic interactions leading to the deposition of materials, the nature of atomic bonding, and the atomic processes and the resulting morphology of film formation. This investigation includes high-resolution core-level spectroscopy work carried out at synchrotron radiation facilities, and much effort has been directed toward the development of photoelectron holography techniques for site-selected 3-D imaging of atomic bonding configurations.

This research is a study of the growth behavior and properties of metal surfaces, interfaces, overlayers, quantum wells, superlattices, and other multilayer structures. The purpose is to obtain a fundamental understanding of the scientific principles that govern the crystalline form, chemical composition, and electronic properties of metal epitaxial systems and quantum structures. The experimental techniques employed in this research include synchrotron photoemission, Auger spectroscopy, electron diffraction, molecular beam epitaxy, and scanning tunneling microscopy and spectroscopy.

This research project is a study of the properties of surfaces, adsorbates, and overlayers using scanning tunneling microscopy/spectroscopy and synchrotron photo emission. We will determine the adsorption site geometry and the overlayer growth mode, and study the modifications to the surface electronic states and the evolution of the interface/overlayer properties during adsorption and overlayer formation. Effects of surface defects, impurities, and atomic steps will be investigated.

X-ray diffraction studies of the atomic structure of surfaces, interfaces, and thin films are being carried out at the National Synchrotron Light Source of the Brookhaven National Laboratory. Current emphasis of this research is on the chemical reactions and atomic rearrangements near an interface when a thin film deposited on a substrate is subjected to high-temperature annealing. The atomic coordinates are determined by Patterson analyses of the in-plane diffraction intensity distributions, by truncation rod scans, and by reflectivity measurements.

We are studying the optical properties of the high-T_c cuprates in order to better understand both the strongly correlated normal state and the nature of quasi-particle pairing in the superconducting state. We are also interested in the unconventional charge dynamics perpendicular to the highly conducting layers in these materials, and are using optical techniques to study the nature of interlayer coupling and c-axis charge transport in the cuprates.

Quantum wire structures provide a ``laboratory'' for studying the effects of low dimensionality, quantum confinement, and superstructure on electronic and vibrational properties. We are applying a variety of optical techniques to study these systems, including time-resolved photoluminescence to explore the relaxational dynamics of photoexcited carriers, Raman scattering studies to investigate confinement and interface effects on phonons, and photoluminescence excitation and infrared absorption to look for confined electronic states.

We are using penetration depth measurements to study the pairing state in high-temperature superconductors. Nonlinear corrections to the current-velocity relation in superconductors are sensitive to the symmetry of the order parameter. These corrections make the penetration depth depend weakly upon an applied magnetic field. By measuring the field and temperature dependence of the penetration depth at low temperatures, we can test for the existence of nodes in the energy gap.

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. Microwave measurements are planned to search for harmonic generation resulting from the presence of nodes in the d-wave energy gap.

This project is a joint effort to develop the fabrication and measurement techniques to study thermoelectric transport processes in nanostructures. Two-dimensional electron gas devices are fabricated from high mobility GaAs/AlGaAs heterojunctions using electron beam lithography. Temperature differences over micron length scales can be generated and detected using quantum point contacts and hybrid quantum wire-dot structures. These techniques offer a possible way to observe the one-dimensional interacting electron liquid, otherwise known as the Luttinger liquid.

We are studying thermoelectric transport phenomena in quantum dots, wires, and point contacts. Measurements are performed in the temperature range 0.05 to 4.2 Kelvin. The transport coefficients in these quantum-scale devices depend strongly upon the conduction of charge and heat through single electron channels. We are interested in both the basic heat transfer processes in submicron devices as well as possible applications for ultrasensitive microwave detection.

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.

We use transmission electron microscopy to examine and control the initial stages of materials growth, primarily in situ. We observe nucleation and surface structure for very thin films and the agglomeration of small particles into nanophase materials.

GaN is important for blue light-emitting devices. This research focuses on understanding the initial stages of GaN growth and the enhancement of growth with energetic nitrogen beam deposition.

We produce and characterize high-temperature superconducting compounds and measure their properties. Our characterization methods include magnetic susceptibility, optical microscopy, and x-ray diffraction. We determine the effect of replacing copper atoms at specific crystallographic sites by other atoms. This provides an important test of the quantum-mechanical theories of these fascinating materials, and helps interpret various data that we and others are obtaining.

We grow and characterize single crystals of superconducting materials and investigate their properties. We want to understand why they are superconducting at such high temperatures, why the critical magnetic field of these materials is highly anisotropic, and how this anisotropy can be related to fundamental parameters of these materials. We determine the superconducting transition temperature and critical magnetic fields. We measure resistance and Hall effect vs. temperature to obtain fundamental information about thermodynamic fluctuations near this second-order transition. Our collaborators measure Raman scattering, spin resonance, and nuclear resonance.

We develop new methods of making high-temperature superconductors to provide single crystals of very homogeneous materials with sharp superconducting transitions and good surface properties. The materials are characterized by x-ray diffraction as well as by measurements of their superconducting properties. The samples produced are used in fundamental experiments by our group and by the other groups in the Science and Technology Center for Superconductivity. The aim of these experiments is to help determine the microscopic phenomena responsible for macroscopic properties. We also do electron microscopy to explore the role of lattice defects in the pinning of quantized magnetic vortices.

The effect of viscosity on dislocation tunneling through impurity pinning points will be determined by comparing the temperature dependence of the ultrasonically found microyield stress in the normal and superconducting state in dilute aluminum alloys. Also, a critical test of the interstitialcy model of condensed matter states will be made by measuring the temperature dependence of the elastic constants of crystals just below the melting temperature. Positive results for an unusual and unexpected effect 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.

Reliable film growth, electronic transport, magnetization measurements, and superconductive tunneling provide the foundation for our investigations into the electronic properties of high-termperature superconductors. Thin films of YBa₂Cu₃O₇ are grown both pure and with atomic substitutions. These thin films are also grown in various crystallographic orientations, allowing measurements of the electronic transport and tunneling properties along different lattice directions in this highly anisotropic material. Through these measurements, information on the interface properties and on the nature of the superconducting mechanism of these materials is being provided.

Properties of high-temperature superconducting thin films are studied for their promise in passive and active device applications. Structural, electronic transport and magnetic properties of MOCVD-grown Bi₂Sr₂Ca₁Cu₂O₈ thin films and Bi₂Sr₂Ca₁Cu₂O₈/Bi₂Sr₂Cu₁O₆ bilayers are studied. The electronic transport characteristics of these structures are investigated as a function of growth morphology and electron-beam induced disorder. The fabrication of Josephson junction and tunnel junction by new techniques and the corresponding junction transport characteristics are being investigated.

Experimental research is directed at measuring and understanding electronic transport across interfaces to high-temperature superconductors. These experiments are performed on four different crystallographic faces of the high-temperature superconductor YBa₂Cu₃O₇. Large-area crystallographic faces are obtained by the growth of high-quality, crystallographically oriented superconducting thin films. The films are grown and layered with a variety of materials, including insulators and normal metals. Our investigations of quasi-particle transport across the high-temperature superconducting interface will help us to understand the proximity effect or the leakage of superconducting pairs into a juxtaposed normal metal.

The project is directed at measuring and understanding electronic and spin transport across interfaces to both low- and high-temperature superconductors. Using ferromagnetic contact electrodes to inject spin-polarized current, spin diffusion and relaxation times are directly probed. In low-temperature superconductors, the injection and detection of spin-polarized quasi-particles in superconducting Nb resulted in the first observation of a nonequilibrium superconducting steady state derived from an injected current of spin-polarized quasi-particles. In high-temperature superconductors we plan to probe the spin-transport across the superconducting interface both parallel and perpendicular to the c-axis in YBa₂Cu₃O₇.

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, and magne tization measurements are conducted in planar microfabricated structures of high-quality niobium thin films grown directly on III-V semiconductors. In these structures, details of the superconducting proximity effect and Andreev reflection are investigated. We have performed the first optical detection of the superconducting proximity effect by the use of Raman spectroscopy as an optical probe of the InAs interface that is in good electrical contact with superconducting niobium.

The principle goal of this work is to identify those excitations that occur within the main electron band, comprising an antibonding combination of copper 3d and oxygen 2p orbitals, and those excitations which involve other orbitals. Strong electron-electron correlations occurring within this band produce antiferromagnetic order in the insulating parent phases. We systematically study two types of excitation: (1) flipping of the spin directions of two neighboring spins (two-magnon excitation main band) and (2) transfer of an electron from one site to a neighboring site (charge-transfer excitation both main band and nonmain band).

Systematics of the effect of changes in doping and stoichiometry on the Raman spectra of high-temperature superconductors (HTS) are investigated. Phonon features, the two-magnon spectrum, and the unusual electronic continuum are of particular interest. Emphasis has been placed on the two-magnon spectrum in doped, superconducting materials. Resonance Raman scattering is used to identify the fundamental charge-transfer excitation in these materials for comparison with that in the insulator.

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 hetero structures, such as Sn/Ge(100) and Ge/Si(100).

X-ray diffraction experiments are carried out in ultrahigh vacuum on a custom-built diffractometer operating on the X16A beamline at NSLS. We prepare the samples on-line and measure the time dependence of their diffraction patterns during deposition. The lineshape tells us about the shape and size distribution of structures that form. The properties of these structures depend on growth rate, coverage, and temperature. When new stable or metastable structures are discovered, we can perform crystallographic determinations. At present we are examining the Cu/Cu(110), Pt/Pt(110), and Pd/Si(111) systems.

We have constructed a new 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 corrosion.

Nanocrystalline materials are prepared either by direct condensation of the constituent materials, by codeposition of immiscible elements followed by annealing to produce precipitated particles, or by layer-by-layer growth. The emphasis in the past year has been on the relationship among the electrical resistance, thermal conductivity, and thermoelectric power of nanocrystalline Fe and Co in Ag and Cu matrices in the concentration range in which large changes in electronic properties occur upon magnetization. A new effort explores the electrical and thermopower of oxide materials that exhibit so-called colossal changes in resistance on the application of magnetic fields.

Metallic superlattices with a high degree of crystalline perfection are produced by molecular beam epitaxy. A rich variety of new properties has been found in Dy/Lu superlattices, which are distinctly different from those of the Dy/Y and Er/Y systems. Recent work has also addressed the behavior of single magnetic layers grown epitaxially and of Nd/Y superlattices. A key element is the effect of strains induced by epitaxy on the magnetic phase diagram. It has proven possible to detect the induced polarization of nonmagnetic constituents of rare-earth alloys using resonant x-ray scattering. Magnetic susceptibility, neutron scattering, x-ray diffraction, and Raman and Brillouin scattering techniques are used.

Several aspects of high-temperature superconductivity are under study. The primary area of interest is the thermal conductivity of untwinned samples of YBa₂Cu₃O_7-x, of YBa₂Cu₄O₈, and of Tl-based superconductors, especially the effect of magnetic fields. A novel anisotropy in the thermal conductivity has been discovered that is induced by magnetic fields applied in the CuO planes and gives evidence for modes in the superconducting gap along certain directions. Some final work on the critical behavior near T^c has been completed.

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) Charge density waves (NMR of NbSe₃) including study of the motion under applied electric fields. Surfaces (1) Electronic properties of the surface layer of atoms of Pt particles, by ¹⁹⁵Pt NMR. (2) Quantum effects arising from the small size of the metal particles. (3) Bonding and structure of molecules (e.g., CO, C₂H₂) adsorbed on Pt, by ¹³C NMR. (4) Special methods: ¹H, ¹³C double resonance to monitor breaking of the C-H bond.

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 YBa₂Cu₃O_7-d or La_2-x dr_aCuO₄, to learn how to describe the normal state, what mechanism leads to superconductivity, and why the transition temperatures are so high. The resonances of ^63,65 Cu, ¹⁷O, ⁸⁹Y, ^135,137 Ba permit NMR to probe specific atomic sites (e.g., Cu nuclei in the CuO₂ planes).

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.

Excitations in condensed matter systems have characteristic properties that are summarized in the dynamic structure factor S ( Q, E ), where Q E are the momentum and energy transfers, respectively, in a scattering process. Insertion devices at synchrotron sources now provide enough x-ray flux to measure S ( Q, E ) from very weakly scattering systems, such as solid helium. In hcp ⁴He electronic excitations are being measured over a range 70 eV and phonon excitations studied with a resolution as small as 9 meV, using specialized spectrometers on the HASYLAB wiggler HARWI at DESY/Hamburg. Extension of these techniques to study interfaces and surfaces appears possible.

Experiments are underway to determine the symmetry of the superconducting pairing state of two exotic superconducting materials the heavy fermion superconductors and the organic superconductors. Both of these are suspected to have unconventional pairing mechanisms that lead to anisotropic energy gap structure. By measuring the magnetic response of single Josephson junctions and dc SQUIDs fabricated between single crystals of the exotic superconductor (the heavy fermion superconductor UPt₃, or the organic system k-ET₂Cu[N(CN)₂]Br) and conventional superconducting thin films, we can determine the relative phase of the order parameter in different directions and thus can distinguish proposed anisotropic pairing states.

Magnetic vortices dominate the thermodynamics and transport properties of microfabricated superconductor arrays and two-dimensional superconducting films. We are studying the motion, pinning, and interactions of vortices by a combination of experimental techniques and computer simulations. The standard approach is to measure magnetotransport properties that reflect the averaged vortex dynamics. In addition, we have developed a scanning SQUID microscope system that allows us to image directly the spatial arrangement and monitor the dynamics of discrete vortices. Images of the configuration of vortices in large arrays and clusters have been obtained and analyzed in terms of vortex diffusion and trapping models.

We are carrying out a series of experiments designed to determine the symmetry of the superconducting pairing state of the high-temperature superconductors. The work involves measurements of the magnetic response of single Josephson junctions and dc SQUIDs fabricated between single crystals of YBCO and conventional superconducting thin films. The experiment is sensitive to the relative phase of the order parameter in different directions and thus can distinguish proposed anisotropic pairing states from the conventional isotropic pairing. Our results give strong evidence for d-wave pairing in YBCO as has been pre- dicted for electron coupling via magnetic spin fluctuations. Work is underway to extend this to other materials and configurations.

We are developing a novel structural and magnetic imaging scanning instrument for studying vortices in high-temperature superconductors. The scanning junction microscope uses a modified STM to scan a single Josephson junction over the surface of a superconducting sample to detect the local magnetic field distribution with submicron resolution. Our goal is to image simultaneously the surface topography and the location of magnetic vortices in the sample. With this capability, we can determine the effect of grain boundaries and defects on the motion and pinning of vortices. We also hope to probe the energy level spectra of quasi-particle bound states in the core of vortices in the cuprates.

We use scanning tunneling microscopy (STM) to investigate confined quasi-particle states in inhomogeneous superconductor structures. STM is a powerful tool that can simultaneously determine the surface structure and give information on the local electronic density of states. We have observed quasi-particle states bound in a normal metal island on top of a superconductor the STM detects the spatial variation and energy dependence of the tunneling conductance. These experiments give information about the interface between superconductors and normal metals and the dynamics of quasi-particles and the proximity effect. These measurements will be extended to study charge transport in semiconductor-superconductor interfaces.

Most conducting materials exhibit conductivity fluctuations with a spectral density approximately inversely proportional to frequency over a wide range. This simple, universal appearance hides a multitude of different mechanisms that can provide information on dynamical properties of many condensed materials, especially ones with significant disorder. We are using this noise to study the dynamics of magnetic vortices in superconductors, domain dynamics in magnets, and basic problems in the formation of glasses and spin glasses.

Many of the dynamical properties of amorphous materials are not accessible to ordinary measurements in large samples, because most of the detailed behavior of individual sites is lost in an average over many differing sites. Ideally, one wants a method to study individual sites at which, for example, atoms move or spins flip. We are now using the statistics of conductance noise to study the fundamental mechanism of the ``colossal magnetoresistance'' effect. We have just developed a new dielectric noise technique, which we are applying to understand a class of unusual dielectrics, called relaxor ferroelectrics, with puzzling and potentially useful properties.

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 attempting to observe Bose-Einstein condensation. The techniques used include time- and space-resolved photolumines cence following laser pulses of nanosecond and picosecond duration.

The propagation and scattering of high-frequency elastic waves (10¹²Hz) in crystals at low temperatures is investigated by ballistic phonon imaging, which produces a spatial map of the heat flux from a point source. Highly anisotropic heat patterns are characteristic of ballistic heat flow at low temperatures. The technique is being used to examine phonon scattering from defects and interfaces. The evolution of thermal energy following intense laser excitation is studied. Processes involved include the frequency down-conversion of terahertz phonons and their interaction with the photo-produced electron-hole plasma. An analogous imaging technique using ultrasonic pulses is being developed. Images of ``internal diffraction'' of ultrasound have been predicted and observed for the first time.

Photoexcitation of GaAs/AlGaAs quantum wells produces nonequilibrium electron-hole pairs with a lifetime of about 1 nsec. These carriers form excitons (at low densities) and electron-hole plasmas (at high densities), which propagate in the confining plane of the GaAs quantum well. We have devised a picosecond pump-probe technique to directly time-resolve the spatial motion of these carriers away from the excitation region. The diffusion and/or drift rates reveal the microscopic scattering processes and forces on the electron-hole pairs in the two-dimensional structures. Evidence for excitonic complexes in quantum wells has been obtained by picosecond photoluminescence imaging.

* Denotes principal investigator.