Bulletin of the American Physical Society
2011 Annual Meeting of the California-Nevada Section of the APS
Volume 56, Number 14
Friday–Saturday, November 11–12, 2011; Menlo Park, California
Session C3: Condensed Matter Physics II |
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Chair: Hendrik Ohldag, SLAC National Accelerator Laboratory Room: Bldg 51 - Kavli Auditorium |
Friday, November 11, 2011 4:00PM - 4:12PM |
C3.00001: Evidence of local distortions in the filled skutterudite compounds LOs4Sb12 (L = Nd, Pr, Eu) from EXAFS studies Trevor Keiber, Frank Bridges We present a temperature dependent extended x-ray~absorption~fine struture (EXAFS) analysis on skutterudite compounds of the form LOs$_{4}$Sb$_{12}$ for L the rare earth elements Nd, Pr, Eu. ~We find the L-Sb and Os-Sb peaks are well ordered while the X-Os and Os-Os peaks show anomalous behavior for X = Nd and Pr. ~We propose a distortion of the lattice structure to account for these changes.~ [Preview Abstract] |
Friday, November 11, 2011 4:12PM - 4:24PM |
C3.00002: EXAFS Analysis of the Local Structure of Thermoelectric Clathrates Scott Medling, Michael Kozina, Frank Bridges We present local structure studies of clathrates (types I, II, and VIII) using the extended X-ray absorption fine structure (EXAFS) technique. The presence of a rattler atom located in the center of one of the cages in the unit cell is believed to strongly scatter phonons and be the origin on the low thermal conductivity that makes these materials promising for thermoelectric applications. We compare a large number of similar clathrates to understand how the local structure explains the electrical and thermal conductivities. For several compounds, including Ba$_8$Ga$_{16}$Sn$_{30}$, EXAFS analysis shows that the local distances are different than average values found from diffraction and show greatly increased disorder compared to Ba$_8$Ga$_{16}$Ge$_{30}$, suggesting that the cage structure is severely distorted, scattering both phonons and electrons, and accounting for the lower thermoelectric figure of merit. [Preview Abstract] |
Friday, November 11, 2011 4:24PM - 4:36PM |
C3.00003: Probing Bulk Electronic Structure with Hard X-ray Angle-Resolved Photoemission Alexander Gray, Jan Minar, Shigenori Ueda, Juergen Braun, Hubert Ebert, Oscar Dubon, Keisuke Kobayashi, Charles Fadley Traditional ultraviolet and soft x-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard x-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. I will present the first experimental data for hard x-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and (Ga,Mn)As, as a technologically-relevant ferromagnetic semiconductor material to illustrate the potential broad applicability of this new technique. The experimental results are compared to free-electron final-state model calculations and more precise one-step photoemission theory including matrix element effects. Some likely future applications areas are discussed. [Preview Abstract] |
Friday, November 11, 2011 4:36PM - 4:48PM |
C3.00004: Ghost critical field and weak localization phenomena in superconducting Tantalum Nitride films Nicholas Breznay, Aharon Kapitulnik We study the appearance of superconducting fluctuations and weak localization effects in a disordered thin film of Tantalum Nitride using magnetotransport measurements. At temperatures above Tc, we observe a large positive magnetoresistance that is 4 orders of magnitude larger than the predicted classical effect. Well above Tc this behavior is consistent with the magnetic field dependence of localization quantum corrections to the conductivity in the presence of strong spin-orbit scattering. Close to Tc and at low magnetic fields the observed magnetoresistance is well described by recent theories that describe both localization and superconducting fluctuations effects. This analysis allows for quantitative study of the inelastic scattering time and the so-called ghost critical field. [Preview Abstract] |
Friday, November 11, 2011 4:48PM - 5:00PM |
C3.00005: Electron and magnetic instabilities in Betts lattices with next nearest neighbors: exact results Armen Kocharian, Gayanath Fernando, Kun Fang The spontaneous phase transitions and quantum critical points in the repulsive ($U>$0) Hubbard model with nearest and next nearest neighbor coupling ($t_{nnn}$), accompanied by (local) charge and spin density inhomogeneities, are studied by exact diagonalization of isotropic Betts cells (8- and 10-site squares) with periodic boundary conditions. The first order phase separation instabilities are found by monitoring charge and spin gaps under the variation of electron density (doping) and magnetic field in a wide range of interaction strength $U$ and t$_{nnn}$. The coupled opposite spins and paired charge of electrons (holes), complied with Bose-Einstein statistics at zero temperature and moderate $U$, suggests full Bose condensation and coherent pairing of electrons in real space with equal gaps (similar to the unique BCS quasiparticle gap). However, a separate pairing of charge and spin degrees at distinct condensation temperatures offers a new route to superconductivity different from the BCS scenario. The conditions for spin liquid, unsaturated and saturated Nagaoka-like ferromagnetism due to spin-charge separation at large $U$ values are also established. The criteria for enhanced coherent electron pairing and saturated ferromagnetism driven by $t_{nnn}$ are considered. [Preview Abstract] |
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