Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H15: Metals: Compounds, Magnetism |
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Sponsoring Units: DCMP Chair: J. von Pezold, Max-Planck Institut fur Eisenforschung Room: B114 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H15.00001: Physical properties from Na$_{24}$Si$_{136}$ single crystals M. Beekman, W. Schnelle, M. Baitinger, H. Borrmann, K. Meier, A. Datta, Yu. Grin, G.S. Nolas The first investigation of intrinsic physical properties from single crystals of a Na$_{x}$Si$_{136}$ (0 $\le \quad x \quad \le $ 24) member is reported. Clathrate-II Na$_{24}$Si$_{136}$ specimens were prepared by a novel method employing spark plasma treatment of the precursor Na$_{4}$Si$_{4}$. The magnitude and temperature dependence of the electrical and thermal transport is understood in the context of metallic conduction, while magnetic susceptibility data indicate sharp features in the electronic density of states near the Fermi level. An Einstein-like vibrational mode that can be attributed to Na guest ``rattling'' is clearly observed through a pronounced contribution to the heat capacity, and explains the exceptionally large atomic displacement parameter for Na obtained from single crystal X-ray diffraction data. The experimental data are in very good agreement with prior electronic structure calculations. [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H15.00002: Tunneling and Rattling in the Intermetallic Cage Compound VAl$_{10+\delta}$ Douglas Safarik, Tomasz Klimczuk, Darrin Byler, Jason Lashley We have used a variety of thermodynamic and transport measurements to study the low-energy ``rattling mode'' in the so-called ``Einstein solid,'' VAl$_{10+\delta}$ [1]. The rattling is thought to arise from weakly bound Al atoms that occupy the large void at the center of Z$_{16}$ Friaf polyhedra, of which there are eight per unit cell in the VAl$_{10+\delta }$ structure. We find that the temperature dependence of our heat capacity and electrical resistivity data are indeed consistent with an Einstein oscillator-type localized mode. In contrast, we find that the $T$-dependence of our elastic constant and internal friction data deviate significantly from that expected for an Einstein oscillator. Indeed, the moduli data show a local minimum at $\sim $20 K, followed by a broad local maximum centered at $\sim $40 K, whereas in the same temperature range the internal friction data show a peak. Both of these observations strongly suggest that the atom inside the polyhedron undergoes quantum mechanical tunneling. We propose a model for the interatomic potential inside the polyhedron that accounts for both the rattling and tunneling behavior. \\[4pt] [1] A. D. Caplin, G. Gr\"{u}ner, and J. B. Dunlop, Phys. Rev. Lett. \textbf{30}, 1138 (1973). [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H15.00003: Lateral photovoltaic effect in Bi$_{2}$Te$_{3}$ James P. Hinton, Jake D. Koralek, Joseph Orenstein, Shane A. Cybart, James Analytis, Ian Fisher We report the observation of a lateral photovoltaic effect (LPVE) in strongly metallic single crystals of Bi$_{2}$Te$_{3}$. In our measurements a focused pulsed laser at 800nm wavelength generates a photovoltage in the absence of applied bias. The sign of the LPVE is independent of the polarization state of the light, as well as the position of the beam spot. The photovoltage is essentially zero at room temperature and has a sharp onset at 190 K. This onset T is closely correlated with a sharp peak in resistivity, of the type that has been associated in the literature with impurity band conduction. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H15.00004: Infrared reflectance of single crystal bismuth in high magnetic fields: strongly correlated physics in the extreme quantum limit Adam Koncz, Andrew LaForge, Zhiqiang Li, Alex Frenzel, Brennan Pursley, Tao Lin, Xinfei Liu, Jing Shi, Sasa Dordevic, Dimitri Basov Infrared reflectance spectra of single crystal bismuth have been measured in magnetic fields as high as 18 T, both parallel and perpendicular to the trigonal axis. Low-field scans with small field steps have produced detailed frequency-field maps of the rich Landau-level transition spectrum. At high fields we observe a dramatic reconstruction of the far-infrared response as quantum confinement effects become dominant. We will discuss these results in the larger context of understanding the quantum-limit physics of strongly interacting electrons in a three-dimensional system. [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H15.00005: Band structure of a superconducting topological metal Lewis Wray, Hsin Lin, Arun Bansil, Yewsan Hor, Robert Cava, Zahid Hasan The Cu$_x$Bi$_2$Se$_3$ system is a topological insulator that has recently been shown to undergo a superconducting transition upon copper doping, raising the possibility that it may be the first known ``topological superconductor". We use angle resolved photoemission spectroscopy to examine the band structure at several copper doping values, observing evolution of the bulk and surface state conduction bands. The addition of copper is found to result in nonlinear electron doping and strong renormalization of the surface state. Characteristics of the three dimensional bulk band structure are discussed in detail with respect to the superconducting state and topological properties. Six-fold rotational anisotropy is evident in the doped conduction bands, with important implications for low energy interactions and potential device development. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H15.00006: The topological insulator and a parasitic metal - friends or foes? Doron Bergman, Gil Refael The best experimental characterization of a topological insulator currently available is by identifying its surface states using ARPES. The same surface states revealed by spin-polarized ARPES in the BiSb topological insulator alloy seem to appear in pure Sb as well (D. Hsieh et al. Science 323, 919 (2009)), and seem to be robust to at least mild disordering of the material. Motivated by this observation, we explore the physics of a topological insulator perturbed by a (parasitic) metallic band, in order to determine whether a bulk metal can have a topologically non-trivial character, and how general these circumstances are. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H15.00007: Optical study of the metal-insulator transition in Bi$_{1-x}$Sb$_{x}$ alloys Dirk van der Marel, Florence Levy, Julien Levallois, Alexey Kuzmenko, Enrico Giannini, Zahid Hassan, David Hsieh Bi$_{1-x}$Sb$_{x}$ exhibits rich physics related to fact that it is an exactly charge compensated plasma of electrons and holes. The electronic structure is controlled by a massive hole band and a 3 dimensional Dirac cone with a gap. The latter gap can be tuned through zero by controlling the Sb concentration, which also results in a relative shift of the bands. Samples can be prepared which are either semi-metallic, semiconducting, or insulating in the bulk with a topological conducting phase at the surface. ~ Detailed optical studies have been realized by means of infrared reflectivity on four different Bi$_{1-x}$Sb$_{x}$ alloys doped at 1{\%}, 6.5{\%}, 7{\%} and 10{\%}. This systematic study demonstrates that chemical pressure tuning induces a zero-temperature metal-insulator transition and reveals a spontaneous phase separation for the 6.5{\%} doped sample. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H15.00008: Anomalous Properties of Single Crystal Niobium Richard K. Bollinger, Ben D. White, John J. Neumeier, Yoko Suzuki, Jon Betts, Albert Migliori, Hugo R. Z. Sandim, Carlos dos Santos A number of anomalous properties have been observed in niobium over the years. The most striking, perhaps, is the anisotropic upper critical field, H$_{c2}$, in the superconducting state. Nb has also been observed to exhibit a ``peak effect,'' a peak in the critical current near T$_{c}$. Experimental evidence will be presented that identifies the source of some of these anomalies as due to interstitial hydrogen at concentrations of a few percent. Thermal expansion, specific heat, resistivity versus temperature, and other data will be presented. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H15.00009: Magnetoresistivity of V$_{3}$Si single crystal: Deviations from K\"{o}hler's Rule and details of the Martensitic transformation Sunhee Moraes, Albert A. Gapud, Peter Favreau, James R. Thompson, David K. Christen The effect of the Martensitic transformation on the field dependence of the normal-state transport magnetoresistivity \textit{$\rho $(H) }in high-quality samples of A15 compound V$_{3}$Si is studied, in the context of the traditional Kohler's Rule, \textit{$\Delta \rho $/$\rho $}$_{0 }$\textit{= f(H/$\rho $}$_{0}$\textit{) = A(H/$\rho $}$_{0})^{b}, $where $A $and $b$ are constants. Contrary to the results of a previous study by Zotos \textit{et al. }on polycrystalline samples (\textit{Sol. State Comm. }\textbf{50 }(5), 1984, p. 453) which found that \textit{b $\sim $ }1.7 in the vicinity of the Martensitic transformation temperature $T_{m}$, the current study on a single crystal finds \textit{two} regimes for $b $on either side of \textit{H/$\rho $}$_{0} \quad \sim $ 0.3 T/($\mu \Omega $cm): For lower fields, \textit{b $\sim $ }0.8; while \textit{b $\sim $ }1.4 for higher fields up to 9 T. There is also indication of a sharp deviation in a very small window around $T_{m}$. These results could reveal more details about the Martensitic transformation, including the possible role of strain fields, as will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H15.00010: Local moments and hyperfine magnetic fields in rare-earth intermetallics XRu$_2$ Dermot Coffey Electronic properties of a series of ferromagnetically ordered rare earth intermetallics, XRu$_2$, are investigated using first principles density functional calculations. The $f$-moment localized on the rare-earth site is found to be determined by the location of the Fermi level as it moves through the majority band for the lighter members of the series and then through the minority band for the heavier members. The interaction between the local moments is mediated by polarized conduction bands derived from the 4$d$ Ru electrons. The size of the hyperfine magnetic fields on the rare-earth sites is correlated with the size of $f$-moments. In contrast there are almost no moments and very small transferred hyperfine magnetic fields on the Ru sites because their 4$d$-electrons are delocalized. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H15.00011: Temperature-dependence of the lattice dynamics of Cr across its magnetic transitions Ruqing Xu, Mary Upton, Ahmet Alatas, Bogdan Leu, Tai-Chang Chiang Chromium is a classic spin-density-wave system. Its phonon dispersion relations at room temperature show a number of anomalies, which can be related to the details of the spin-dependent electronic structure. As the temperature is varied, Cr undergoes two magnetic transitions, one at 311 K, and the other at 123 K, and the elastic constants are known to change abruptly across these transitions. Thus far, there has been very limited information about the behavior of the phonon dispersion relations across these transitions, but large changes appear likely. We have employed high-resolution inelastic x-ray scattering to address this question. The experiment was carried out at Sector 3 of the Advanced Photon Source. Surprisingly, we have found no obvious abrupt changes in the phonon dispersion relations across the magnetic transitions. The phonon frequencies remain largely the same for sample temperatures down to 30 K. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H15.00012: Magnetism effects on structural properties in Al- and Si- substituted Laves phases Fe$_{2}$Nb and Fe$_{2}$W Fran{\c c}ois Liot, Martin Fri{\'a}k, J{\"o}rg Neugebauer Laves phases are promising candidates for the design of new steels with superior mechanical strength. Here we study systematically thermodynamic, structural and magnetic properties in Al- and Si- substituted Laves phase compounds Fe$_{2}$Nb and Fe$_{2}$W using first-principles density functional methods. Spin polarized calculations predict that substituting Al atoms for Fe atoms in Fe$_{2}$Nb is energetically more favorable than substituting Al atoms for Nb atoms. Furthermore, they show that this leads to a significant increase of the lattice constant along the \emph{a} axis. To investigate the effects of magnetism, non-magnetic calculations are carried out for the same Fe-Nb-Al compounds. Theoretical predictions for the other alloys series, Fe-Nb-Si, Fe-W-Al and Fe-W-Si are presented. This work emphasizes the necessity of taking into account magnetism to understand the structural properties of the ternary alloys. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H15.00013: Electronic structure and quantum critical behavior of NbFe$_{2}$ Brian Neal, Erik Ylvisaker, Warren Pickett The C14 hexagonal Laves phase compound NbFe$_{2}$ sits on the edge of a magnetic instability. By varying the composition, Nb$_{1-y}$Fe$_{2+y}$ encompasses two ferromagnetic states, an antiferromagnetic or spin density wave state, and a quantum critical point (y = -0.015). The electronic structure has been studied using density functional methods. The energy-minimizing structural parameters significantly differ from experimental values. We investigate a flat region in the band structure near the Fermi energy, for possible connection to quantum criticality. Fixed spin moment calculations reveal the character of magnetic moments at the two distinct Fe sites. [Preview Abstract] |
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