Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X15: Structural and Electronic Properties of Metals I |
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Sponsoring Units: DCMP Chair: Di Xiao, Oak Ridge National Laboratory Room: B114 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X15.00001: Confinement-induced Berry phase and helicity-dependent photocurrents J. Orenstein, Joel E. Moore The photocurrent in an optically active metal is known to contain a component that switches sign with the helicity of the incident radiation. At low frequencies, this current depends on the orbital Berry phase of the Bloch electrons via the ``anomalous velocity'' of Karplus and Luttinger. We consider quantum wells in which the parent material, such as GaAs, is not optically active and the relevant Berry phase only arises as a result of quantum confinement. Using an envelope approximation that is supported by numerical tight-binding results, it is shown that the Berry phase contribution is determined for realistic wells by a cubic Berry phase intrinsic to the bulk material, the well width, and the well direction. These results for the magnitude of the Berry-phase effect suggest that it may already have been observed in quantum well experiments. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X15.00002: Momentum selective imaging of electrons in orbit: autonomous current scanning tunneling microscopy Joonhee Lee, Shawn Perdue, Alejandro Rodriguez Perez, Vartkess Ara Apkarian The back-action due to inelastic electron tunneling generates detectable motion of the scanning tunneling microscope (STM) tip, which in turn modulates current. This feedback leads to parametric oscillations of the tip, observable as autonomous current. The modulation frequencies are specific to the angular momentum of the tunneling electron. The principle can be used to image atomic and molecular eigenstates (orbitals) with angular momentum selectivity, and with phase selectivity in the case of electronic superposition states. Captured images of the orbitals of a Ag atom and a Ag-dimer adsorbed on a NiAl(110) surface will be presented to illustrate the principles that allow the recording of images of orbitals in motion, without using timing devices. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X15.00003: Search for multiple-electron emission in Auger transition processes in solids S. Kalaskar, S.L. Hulbert, Q. Dong, B.A. Bartynski, A.H. Weiss We present electron-electron coincidence measurements from Ag(100) taken using a synchrotron radiation photon beam of 465eV energy (which is just above the Ag 3d threshold), with one electron energy analyzer fixed at 175 eV kinetic energy and the other scanned from 150 to 200 eV. The data show a pronounced step at 175 eV consistent with processes in which the energy associated with the filling of the M core hole is shared with two or more correlated electrons that are emitted in an Auger transition accompanied by multiple-electron emission. These results provide direct evidence for the existence of these multiple-electron Auger processes, first posited to explain the origin of the large low energy tail characteristic of Auger spectra from solids.\footnote{E. Jensen, R. A. Bartynski, R. F. Garrett, S. L. Hulbert, E. D. Johnson, and C.-C., Phys. Rev. B 45, 13636 (1992)} [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X15.00004: Nonequilibrium Band Mapping of Unoccupied Bulk States Below the Vacuum Level by Two-Photon Photoemission Jerry I. Dadap, Zhaofeng Hao, Kevin R. Knox, Mehmet Yilmaz, Nader Zaki, Richard M. Osgood Jr., Peter D. Johnson We demonstrate two-photon photoemission (2PPE) to map a bulk unoccupied band, \textit{viz.} the Cu \textit{sp}-band 0 to 1 eV below the vacuum level, in the vicinity of the L point. This short-lived bulk band is seen due to the strong optical pump rate, and the observed transition energies and their dispersion with photon energy, are in excellent agreement with tight-binding band-structure calculations. A careful analysis of the 2PPE spectra permits unambiguous assignment of the transitions. The variation of the 2PPE final-state energy with photon energy has a measured slope of $\sim $1.64, in contrast with values 1 or 2 observed for 2PPE from two-dimensional states, i.e., image and surface states. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X15.00005: Density Functional Theory for Excited States Antonios Gonis, Don M. Nicholson, X.-G. Zhang, G. Malcolm Stocks, Simone Chiesa Using the concept of entangled states it is shown that density functional theory in its initial ground-state formulation\footnote{P. Hohenberg and W. Kohn, Phys. Rev. \textbf{136}, B864, (1964).}$^,$\footnote{W. Kohn and L. J. Sham, Phys. Rev. \textbf{140}, A1133 (1965).}$^,$\footnote{M. Levy, Proc. Nat. Acad. Sci. USA \textbf{76}, 6062 (1979).} is inherently capable of treating the excited states of interacting many-particle systems.\footnote{\textbf{Acknowledgement}: Discussions with Mel Levy are gratefully acknowledged. This work was performed under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344 with LLNS, LLC. Work at ORNL was performed under the auspices of the Division of Materials Science and Engineering and at CNMS by the Division of Scientific User Facilities, Office of Basic Energy Science of the US Department of Energy.} [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X15.00006: Finite-Temperature Predictions of Inhomogeneous Chemical Correlations using Self-consistent KKR-DCA Dominic A. Biava, Duane D. Johnson The optimal-basis\footnote{Phys. Rev. B 80, 125123 (2009)} Korringa-Kohn-Rostoker (KKR) electronics-structure method combined with the dynamical cluster approximation (DCA) provides a first-principles framework to predict accurately the electronic, structural, chemical short-ranged order (SRO) effects in disordered alloys. We perform subspace restricted, ensemble averaging over Betts\footnote{Can. J. Phys. Rev. 75, 47-66 (1997)} clusters with probability distributions preserving higher-order sum rules and show that the resulting electronic grand potential has simple dependence on SRO. We then predict directly the chemical SRO by minimizing the electronic grand potential with chemical entropy with respect to the probability distribution at temperature. We correlate the atomic degrees of freedom over Betts$^2$ clusters and investigate the electronic and structural effects arising from the SRO in various disordered alloys, showing good agreement to observed SRO. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X15.00007: Spectral phase characterization of laser induced coherent vibration and its applications in Raman spectroscopy Xiaoji Xu, Stanislav Konorov, Yakun Chen, Valery MIlner, John Hepburn The spectral phase of laser induced molecular vibration in the coherent Raman process carries additional information about the molecular structure and dynamics of relaxation. Through a novel method of amplitude and phase characterization and reconstruction, experimental study on liquid ethanol sample shows an out of phase behavior between two set of resonances levels. The result demonstrated the ability to resolve the signal of the Raman susceptibility that is otherwise not available in spontaneous Raman spectroscopy. An interpretation of out of phase behavior between resonances is made using Density Functional Theory (DFT) calculation, and the result is found to be correlated with the symmetry of molecule. This study raises the awareness of symmetry-based intrinsic excitation phase difference in ultrafast coherent Raman spectroscopy on its potential applications to molecular orientation studies. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X15.00008: Temperature fluctuations and their statistics in electron systems out of equilibrium Tero Heikkil\"a, Matti Laakso, Yuli Nazarov We study the fluctuations of the electron temperature in a metallic island coupled to reservoirs via resistive contacts and driven out of equilibrium by either a temperature or voltage difference between the reservoirs in the regime in which the electrons are completely decoupled from the lattice phonons. We quantify these fluctuations in the regime beyond the Gaussian approximation and elucidate their dependence on the nature of the electronic contacts.\footnote{Tero T. Heikkil\"a and Yuli V. Nazarov, Phys. Rev. Lett. {\bf 102}, 130605 (2009).} Besides normal noninteracting contacts, we also study the temperature fluctuations and their effects on other transport properties in single-electron transistors. We find three distinct regimes corresponding to cotunneling, sequential tunneling, and their coexistence. We find that the Fano factor of current fluctuations is enhanced around the crossover from coexistence to sequential tunneling by several orders of magnitude. This is because the SET is very sensitive to temperature fluctuations around this crossover. We also study the statistics of temperature fluctuations in these regimes and find the distribution to be strongly non-Gaussian. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X15.00009: Auger-Photoelectron Coincidence Measurements of the Low Energy Tail of Cu MVV and Ag NVV Auger Spectra K. Shastry, S.L. Hulbert, Q.Y. Dong, R.A. Bartynski, A.H. Weiss We present the results of measurements in which Auger photoelectron coincidence spectroscopy (APECS) was used to obtain the energy distribution of electrons emitted as a result of the MVV transition in Cu over the range of 0eV-81eV and the NVV transition in Ag over the range 0eV - 100 eV. A novel differences technique was used together with APECS to eliminate all backgrounds from inelastic processes not directly related to the selected Auger transitions. The measurements reveal a well formed auger peak at 40 and 60 eV for Cu and Ag respectively accompanied by a significant spectral weight in a low energy tail of that extends to 0eV with a characteristic cascade like bump at low energies. We posit that the LET in the Cu and Ag spectrum is due to both extrinsic processes in which Auger electrons emitted with the full energy of the Auger transition lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi- electron Auger processes distribute the transition energy to multiple electrons [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X15.00010: Ab initio calculation of X-ray absorption spectra for warm dense metals Vanina Recoules, St\'ephane Mazevet We developed a first principle approach to calculate the x-ray absorption near edge spectra (XANES) of dense plasmas [1] based on density functional electronic structure calculations and quantum molecular dynamics simulations. This approach provides a first principle consistent description of both the electronic structure and the thermodynamics state of the system by using large supercells. We applied the method to the calculation of the XANES spectra near K-edge of aluminium at solid density for temperatures up to 6eV. We show that XANES spectrum loses its room temperature structure, first due to melting and second due to loss of correlation of the liquid. We obtain a good agreement with measurements of XANES spectra performed for an Al foil isochorically heated. We also applied the method to the calculation of the XANES spectra near K-edge of iron at solid density with temperature up to 1eV.\\[4pt] [1] V. Recoules and S. Mazevet, PRB 80, 064110 (2009) [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X15.00011: Application of the Phase-Field Crystal Model for Quantitative Materials Modeling Tapio Ala-Nissila The recently proposed Phase-Field Crystal (PFC) model [1] can be used to model materials on microscopic length but diffusive time scales. The model can be related to the classical density functional theory of liquids [2] allowing modeling of solidification, phase segregation, grain growth, elastic and plastic deformations in anisotropic systems with multiple crystal orientations. In this talk, I will discuss recent progress on using the PFC model and its extensions to describe quantitative modeling of selected properties of BCC Fe [3]. \\[4pt] [1] K.R. Elder, M. Katakowski, M. Haataja, and M. Grant, Phys. Rev. Lett. {\bf 88}, 245701 (2002); K.R. Elder and M. Grant, Phys. Rev. E {\bf 70}, 051605 (2004).\\[0pt] [2] K.R. Elder, Nikolas Provatas, Joel Berry, Peter Stefanovic, and Martin Grant, Phys. Rev. B {\bf 75}, 064107 (2007).\\[0pt] [3] A. Jaatinen, C. V. Achim, K. R. Elder, and T. Ala-Nissila, Phys. Rev. E {\bf 80}, 031602 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X15.00012: Melting of Simple Solids and the Elementary Excitations of the Communal Entropy Angelo Bongiorno The melting phase transition of simple solids is addressed through the use of atomistic computer simulations. Three transition metals (Ni, Au, and Pt) and a semiconductor (Si) are considered in this study. Iso-enthalpic molecular dynamics simulations are used to compute caloric curves across the solid-to-liquid phase transition of a periodic crystalline system, to construct the free energy function of the solid and liquid phases, and thus to derive the thermodynamical limit of the melting point, latent heat and entropy of fusion of the material. The computational strategy used in this study yields accurate estimates of melting parameters, it consents to determine the superheating and supercooling temperature limits, and it gives access to the atomistic mechanisms mediating the melting process. In particular, it is found that the melting phase transition in simple solids is driven by exchange steps involving a few atoms and preserving the crystalline structure. These self-diffusion phenomena correspond to the elementary excitations of the communal entropy and, as their rate depends on the local material cohesivity, they mediate both the homogeneous and non-homogeneous melting process in simple solids. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X15.00013: Vital roles of the biquadratic interaction on electromagnon excitations Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa We have succeeded in solving the long-standing puzzles of electromagnon excitations in the multiferroic Mn perovskites with specific two peak spectra in the THz frequency regime. We first construct a realistic model for these compounds, which includes frustration among the spin exchanges, a conflict between DM interactions and magnetic anisotropies, as well as the biquadratic interaction originating from the spin-phonon coupling. We reproduce the lower peak spectrum around 3 meV in addition to the higher one around 6-8 meV, and show that the very weak biquadratic interaction is crucially important for generation and enhancement of the lower-lying mode. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X15.00014: Electronic Structure Calculations for Heavy Elements: Radon (Z=86) and Francium (Z=87) Alexander Koufos, Dimitrios Papaconstantopoulos Electronic structure calculations allow scientists to predict the properties of solids without the use of physical material. Although the ability to manipulate matter has improved dramatically within the past couple decades, some matter is still hard to study. Modern computers not only let us study this matter, but allow us to do it more quickly and just as accurately. The electronic structure of two rare and mostly unstudied elements, Radon (Z=86) and Francium (Z=87), has been calculated. The augmented plane wave (APW) method with local density approximation (LDA) functional as well as the linearized augmented plane wave (LAPW) method with both LDA and generalized gradient approximation (GGA) functionals were used to perform the calculations. Francium total energy calculations gave the fcc structure slightly below the bcc structure with a minimal energy difference of $\Delta E=0.33mRy$. The difference found is consistent with other alkali metal total energy calculations which do not verify the bcc structure to be the ground state. Radon was predicted to be an insulator with a gap of $0.931 Ry$ similar to the other noble gases. [Preview Abstract] |
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