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Materials Research Laboratory

Physics: Experimental Condensed Matter Physics
^ 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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 the Magnetic Oxides
S. L. Cooper,* S. Yoon, H. Rho
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ In Situ Studies of Materials Growth
J. M. Gibson,* J. C. Yang, M. Yeadon, W. Henstrom, M. Kleinschmidt
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ Tunnel-Junction Fabrication Using Chemical Techniques
L. H. Greene,* P. Hentges; W. Klemperer, G. Westwood (Chem.)
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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-δ (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.

^ 34ID Beamline Construction
I. K. Robinson,* C. A. Benson
National Science Foundation, DMR 97-24294; U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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

^ Solid-Liquid Interface Studies by X-Ray Diffraction
I. K. Robinson,* J. deVilbiss, A. A. Gewirth
U.S. Department of Energy, DE-FG02-91ER45439, DEAC02-76CH00016; National Synchrotron Light Source

(In cooperation with the Frederick Seitz 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 FS-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, DE-FG02-91ER45439, DEAC02-76CH00016; Advanced Photon Source

(In cooperation with the Frederick Seitz 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.

^ Magnetic Behavior of Oxides and Nanophase Materials
M. B. Salamon,* H. Yanagihara, P. Lin, S. Baily
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ Excitations in Complex Condensed Systems
R. O. Simmons,* D. A. Arms
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ Magnetic Screening and Phase Fluctuations in Underdoped Cuprates
D. J. Van Harlingen,* K. D. Osborn, J. A. Bonetti
U.S. Department of Energy, DE-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.

^ 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-FG02-91ER45439

(In cooperation with the Frederick Seitz 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.


Summary of Engineering Research