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Experimental Condensed Matter Physics
^ Laboratory for Nanoscale Science and Engineering
R. Averback, P. Bellon, M. B. Salamon*
University CRI Program

A new laboratory is being set up to study the synthesis and processing of novel nanostructured materials using far-from-equilibrium methods. Initial research is focused on nanocomposites of magnetically soft and hard materials exhibiting an effect known as exchange-spring behavior. Ball milling at elevated temperatures and ion-beam mixing of epitaxial film are being used to control the interfaces between the two components. This work is coupled with computer simulations.

^ 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 these films, 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. One goal of this project 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.

^ Development of an Ultrahigh Resolution Photoemission System for Studies of Quantum Structures
T. C. Chiang,* T. Miller, D. Luh
U.S. National Science Foundation, DMR 99-75182

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

^ Electronic Properties of Impurities, Surfaces, and Quantum Structures
T. C. Chiang,* T. Miller, D. Luh, M. Upton
National Science Foundation, DMR 99-75470

High-resolution angle resolved photoemission is employed to investigate the 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 a 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, electron-electron scattering, and structural stability will also be investigated.

^ Semiconductor and Ceramic Surfaces and Interfaces
T. C. Chiang,* T. Miller, D. Luh, T. Kidd, M. Holt, D. Ricci, S. Sligh, Z. Wu
U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Photoemission and x-ray diffraction and scattering techniques are employed to determine the electronic properties and atomic structure of surfaces, interfaces, 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 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 behaviors, 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.

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

Researchers are interested in a number of low carrier density magnetic systems that exhibit rich phase diagrams as a function of doping, temperature, and magnetic field, as well as diverse phenomena such as spin polaron formation and field-induced metal-insulator transitions. The diverse phase diagrams and phenomena exhibited by these materials derive largely from the competition between strong Coulomb correlations, electron-phonon coupling, and spin interactions. The research team will use 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. Rho
U.S. Department of Energy, DEFG02-96ER45439

(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. Researchers are attempting to elucidate the physics governing these interesting phase regions by using reflectance, Raman, and Brillouin scattering spectroscopies to study the interactions among the lattice, charge, and spin degrees of freedom in these materials.

^ Growth and Properties of Single-Crystal Films
C. P. Flynn,* M. Ondrejcek, C. Durfee
U.S. Department of Energy, LEEM, DEFG02-96ER45439

(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, and threading dislocations) and bulk morphology (including screw, edge, and interfacial dislocations and subboundaries) can be examined in real time and during actual growth.

^ Field-Dependent Penetration Depth in Unconventional Superconductors
R. W. Giannetta*
National Science Foundation, DMR 01-01872

The focus of this research is on the electromagnetic properties of superconductors whose pairing symmetry is believed to be unconventional. These include both hole and electron-doped cuprates and organic and certain magnetic superconductors. Using high resolution, magnetic field-dependent measurements of the London and Josephson penetration depth researchers study nodal quasiparticles, surface Andreev bound states, vortex motion, and the coexistence of superconductivity and ferromagnetism. The aim is to develop field-dependent penetration depth into a new spectroscopy of the superconducting state.

^ Penetration Depth Measurements in Unconventional Superconductors
R. W. Giannetta*
University of Illinois Research Board

This work utilizes low temperature, radio frequency penetration depth techniques to detect a quantum effect unique to unconventional superconductors: the surface Andreev bound state. The recent electromagnetic detection of bound states provides a contactless probe of the underlying superconductive gap symmetry, applicable in principle to any material. The effect can be enhanced through heavy ion irradiation, making possible the study of inhomogeneous d-wave superconductivity in crystals with nanoscale defects.

^ In Situ Studies of Materials Growth
J. M. Gibson,* J. C. Yang, M. Yeadon, W. Henstrom, M. Kleinschmidt
U.S. Department of Energy, DEFG02-96ER45439

(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 relying on surface-related diffracted intensities. Quantitative atomic resolution microscopy is being applied to interface structure and chemistry.

^ 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.

^ Charge Transport across Superconductor/Semiconductor and Superconductor/Normal-Metal
L. H. Greene,* J. Elenewski, A. C. Abeyta, I. V. Roshchin, W. L. Feldman; P. W. Bohn, T. Tanzer, X. L. Li (Chem.); J. F. Klem (Sandia National Laboratories); G. Spalding (Physics, Illinois Wesleyan Univ.)
U.S. Department of Energy, DEFG02-91ER45439

(In cooperation with the Materials Research Laboratory)

The static and dynamic properties of hybrid superconductor-semiconductor structures are studied. Electronic transport, superconductive tunneling, and light-scattering measurements are conducted on planar, nanofabricated structures of high quality thin films of metallic superconductors grown directly on III-V semiconductors. The superconducting proximity effect, Andreev reflection, and tunneling are investigated. In order to extend previous work on the optical detection of the superconducting proximity effect on Nb/InAs interfaces, researchers have developed a nanosphere lithographic technique to create defect-free colloidal mono- and bi-layers for use as masks. Raman scattering on these nanoscale Nb arrays on InAs are performed.

^ Neutron Scattering and Muon Spin Rotation
L. H. Greene,* E. Dumont; G. Feltcher, S. G. E. te Velthuis (Argonne National Laboratory); H. Keller (Paul Scherrer Institute)
U.S. Department of Energy, DEFG02-91ER45439

(In cooperation with the Materials Research Laboratory and Ministre de la Culture, Luxembourg)

The existence of spontaneously broken time reversal symmetry, or the spontaneous generation of magnetic fields, in the near-surface region of unconventional superconductors is studied by grazing-incidence polarized neutron scattering (GIPoNS) and low-energy muon spin rotation (LEM). The GIPoNS is performed at the IPNS facility at Argonne National Laboratory (ANL) and the LEM is performed at Paul Scherrer Institute (PSI) in Switzerland.

^ Superconductive Tunneling Spectroscopy and Electronic Transport in Pure and Doped YBa2Cu3O7
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-delta (YBCO) 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; W. Klemperer, G. Westwood (Chem.)
U.S. Department of Energy, DEFG02-91ER45439

(In cooperation with the Materials Research Laboratory)

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 surface, but this method damages the surface of the superconductor. To avoid such surface degradation, researchers have developed a fabrication technique in which an ultrathin insulating layer of ZrO2 is condensed onto the surface of YBa2Cu3O7-delta (YBCO) thin films. Researchers find less damage to the YBCO interface than other junction fabrication techniques, and this method passivates the YBCO surface over at least a year's time in laboratory ambient. These junctions are used to study the electrodynamic properties of the Andreev bound state.

^ Tunneling Spectroscopy of High-Temperature and Other Unconventional Superconductors
L. H. Greene,* H. Aubin, X. Yu
National Science Foundation, DMR 99-72087

Researchers 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.

^ Search for Subdominant Order Parameter Phases in d-Wave Grain Boundary Junctions
D. J. Van Harlingen,* W. K. Neils
U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Researchers 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. The research team is 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.

^ Experimental Determination of the Pairing State of Unconventional Superconductors
D. J. Van Harlingen,* J. E. Sadleir
National Science Foundation, DMR 99-72087

Experiments are under way 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, researchers 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.

^ Magnetic Imaging of Vortices in High-Temperature Superconductor Films and Devices
D. J. Van Harlingen,* A. Ruosi
University of Illinois, University Scholar Grant

This project uses 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. Researchers 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, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Researchers 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. The team is 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.

^ Phase Coherence and Dynamics of Pi-Josephson Junctions
D. J. Van Harlingen,* T. Crane, S. Frolov
National Science Foundation, DMR 99-72087

Researchers are fabricating and testing superconductor junctions that may exhibit a pi-phase shift in the Josephson current-phase relation. Two systems are under study: superconductor-ferromagnet-superconductor (S-F-S) junctions, in which the phase shift arises from magnetic scattering in the barrier, and mesoscopic S-N-S junctions, in which the Josephson tunneling can be controlled by inducing a non-equilibrium quasiparticle distribution in the normal layer. A series of SQUID interferometer measurements and current-phase relation measurements are under way.

^ Raman Scattering from Electronic Excitations in Superconductors
M. V. Klein,* H. Rho
Science and Technology Center for Superconductivity

(In cooperation with the Materials Research Laboratory)

It costs an energy greater than the gap 2Δ to create an excitation in a superconductor. In a wide variety of superconductors, this results in redistribution in the intensity of inelastically scattered light into a peak at a photon energy shift of 2Δ. Researchers study this peak as a function of temperature, magnetic field, and impurity concentration in the rare-earth nickel borocarbide family of superconductors, where the rare-earth site is occupied by Y or Lu atoms. With that site occupied by rare-earth atoms, scientists study the interplay between the rare-earth magnetism and superconductivity.

^ 34ID Beamline Construction
I. K. Robinson,* C. A. Benson
National Science Foundation, DMR 97-24294; U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Researchers 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 this 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

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

In these experiments, researchers 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. Researchers are 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. The team expects to be able to image the spatial distribution of strain within grains.

^ Solid-Liquid Interface Studies by X-Ray Diffraction
I. K. Robinson,* J. deVilbiss, A. A. Gewirth
U.S. Department of Energy, DEFG02-96ER45439, DEAC02-76CH00016; National Synchrotron Light Source

(In cooperation with the Materials Research Laboratory)

Researchers have constructed a high-speed diffractometer, employing the "kappa" geometry, for the measurement of electrochemical interfaces. Versions exist on the X16C beamline at NSLS, Brookhaven National Laboratory, and in the basement of the Frederick Seitz Materials Research Laboratory. The beamline has a focused beam of 1010 photons per second in a submillimeter spot on the sample. The MRL setup uses a focusing graphite monochromator with 108 photons per second. Researchers 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. The team is studying metal and ionic adsorbed species on copper surfaces by x-ray diffraction to try to gain an understanding of copper and aluminum corrosion. The structural mechanisms behind electrocatalysis are also studied by the use of appropriate transition state analogs.

^ X-Ray Diffraction Investigations of Protein Crystallization
I. K. Robinson,* S. Boutet
U.S. Department of Energy, DEFG02-96ER45439, DEAC02-76CH00016; Advanced Photon Source

(In cooperation with the Materials Research Laboratory)

Researchers have observed the crystal truncation rods due to diffraction from the outermost monolayer of a crystal of the protein ferritin. These provide information about the surface structure, particularly the question of which facets are present on the crystal. The facets of the crystal that are studied appear naturally in the growth morphology. The initial stages of crystallization of proteins are largely unknown, and yet are of vital importance to the future of biotechnology. With the experience gained from these large crystals, the team is now turning to nanometer-sized crystals that reflect the later stages of nucleation. These are predicted to have very different morphology. They are prepared by freezing crystallization solutions at different stages of growth and are measured with coherent x-ray diffraction at our Argonne facility.

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

University of Illinois researchers 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, researchers 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.

^ Magnetic Behavior of Oxides and Nanophase Materials
M. B. Salamon,* H. Yanagihara, P. Lin, S. Baily
U.S. Department of Energy, DEFG02-96ER45439

(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. Researchers 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 team recently turned attention to other oxides that have similar behavior, but contain iron or cobalt as the magnetic element.

^ Phase Transitions in High-Temperature Superconductors
M. B. Salamon,* T. Park, E. Chia
National Science Foundation, DMR 99-7087 (with D. Van Harlingen, et al.)

This research focuses on efforts to study the nature of unconventional superconductors through the behavior of the superconducting penetration depth at low temperatures. The materials studied include the possible p-wave superconductor Sr2RuO4 and various nickel borocarbides. Further studies of the d-wave nature of the superconducting state in the cuprates are under way, including predictions of a dependence of the heat capacity on the orientation of a magnetic field. Heat capacity studies of the borocarbides and the newly discovered superconductor MgB2 are also being carried out.

^ Excitations in Complex Condensed Systems
R. O. Simmons,* D. A. Arms
U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Prototype complex condensed systems are studied over a wide domain of density, temperature, interactions, and quantum effects. Synchrotron x-ray and epithermal neutron scattering are used, and direct critical comparisons are made with path-integral Monte Carlo and other a priori calculations. Properties of interest include electronic and collective excitations, single-particle dynamics, impurity-matrix interactions, and the equilibria and dynamics of structural phase transformations including patterning and other textural questions. Separately, x-ray diffraction and pressure techniques are applied to study the properties of crystalline defects and intrinsic properties in He isotope crystals.

^ Momentum Densities and Electronic Properties of Noble Gas Solids and Fluids
R. O. Simmons,* A. T. Macrander (Argonne National Laboratory), M. Schwoerer-Bohning (Carnegie Institution), D. N. Timms (Univ. Portsmouth)
U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory, Argonne National Laboratory, and Rutherford Appleton Laboratory, U.K.)

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. Two regimes are of particular interest in this project. The first is neutron scattering in the impulse approximation, which yields atomic momentum densities in fluid and solid He, Ne, Ar, and H2. The second is electronic excitations in these wide band-gap systems, in both solid and fluid form. Synchrotron x-rays can study these excitations over broad energy ranges (100 eV) and in systems under high pressure.

^ Nuclear Magnetic Resonance in Solids
D. Y. Smith, A. Comment, C. P. Slichter,* J. P. Ansermet, C. T. Milling
U.S. Department of Energy, DEFG02-96ER45439

(In cooperation with the Materials Research Laboratory)

Researchers 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). Some examples of solids are high-temperature superconductors, for which NMR provides detailed information about both the normal and superconducting states; organic conductors that exhibit both superconductivity and antiferromagnetism; magnetic solids such as those exhibiting colossal magnetoresistance; and charge density waves (NMR of NbSe3), including study of the motion under applied electric fields. Examples of surfaces include electronic properties of the surface layer of atoms of Pt particles, by 195Pt NMR; bonding and structure of molecules (such as CO, C2H2) adsorbed on Pt, by 13C NMR.

^ Noise Investigations of Condensed Matter Systems
M. B. Weissman,* E. Kolla, 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. Researchers are using this noise to study colossal magnetoresistive effects, relaxor ferroelectrics, Barkhausen effects in magnets, and other problems in condensed matter.


Summary of Engineering Research