Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X22: Metals: Bulk Properties and Nanostructures |
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Sponsoring Units: DCMP Chair: Lin-Lin Wang, Ames Laboratory/Iowa State University Room: D163 |
Thursday, March 24, 2011 2:30PM - 2:42PM |
X22.00001: Phonon self energy in transition metals Laurent Chaput, Atsushi Togo, Isao Tanaka, Gilles Hug We present \emph{ab initio} calculations of the phonon self energy of transition metals obtained using second order many body perturbation theory.\footnote{S. Narasimhan and D. Vanderbilt, Phys. Rev. B, 43, 4541 (1991)} The code we have implemented\footnote{L. Chaput, A. Togo, I. Tanaka and G. Hug, submitted to Phys. Rev. B} use the symmetry properties of the phonon-phonon interactions to express the self energy as a sum over irreducible triplets. It is analogous to the reduction of integration to the irreducible part of the Brillouin zone for one particle properties. The self energy of transition metals is then calculated. We show that the Peierls approximation\footnote{R. E. Peierls, Quantum Theory of Solids, Oxford University Press,1964} is in fact reasonable for \emph{bcc} and \emph{fcc} metals, but fails for the \emph{hcp}. The decays paths of phonons producing the self energy is finally analyzed using surfaces of reciprocal space defined by conservation law. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X22.00002: Observation of phonon softening in Cr near its Neel transition Ruqing Xu, Tai-Chang Chiang Chromium is a classic antiferromagnetic spin-density-wave system, with many unique properties yet to be fully understood despite the extensive experimental and theoretical efforts in the past. For instance, near its two magnetic transitions at 311K and 123K, the elastic constants of Cr have been observed to soften abruptly by ultrasonic experiments, indicating a strong lattice-spin interaction. However, such softening has never been confirmed in previous measurements of Cr's phonon dispersion relations. To address this issue we have carried out studies with inelastic x-ray scattering (IXS) as well as x-ray thermal diffuse scattering (TDS) at temperatures around the Neel transition (311K). While the IXS measurements did not find obvious changes in the overall phonon dispersion relations of Cr, abrupt changes in TDS intensities were clearly observed across the transition at wavevectors close to the Brillouin zone centers, unveiling a softening in the long-wavelength lattice excitations in Cr at the Neel transition. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X22.00003: First-principles study of phonon-phonon interaction in FCC metals at high temperatures Xiaoli Tang, Chen W. Li, Brent Fultz Third-order lattice anharmonicity induced phonon broadening of FCC metals (including Al and noble metals Cu, Ag, Au) were calculated from first-principles density functional theory (DFT) using the second-order perturbation theory, where anharmonic force constants were obtained from supercell finite displacement method combined with DFT calculations. For aluminum, the good agreement between our calculations and prior measurement of phonon linewidth at 300K and our new measurement of phonon density of states to 750K indicates the third-order phonon-phonon interactions can account for the lifetime broadenings of phonons in aluminum to at least 80\% of its melting temperature. A systematic study of noble metals further suggests that, despite of the similarity among these systems, scattering kinematics play an important role in determining the relative anharmonicity between the modes, while potential anharmonicity modulates the absolute phonon decay rate. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X22.00004: Dynamical behavior of coherent phonons in semimetals: Measuring fast electron decoherence rates with slow pulses Jian Chen, Jingjing Li, Stephen Fahy, Roberto Merlin, David Reis Coherent light illumination of solids above the band-gap leads not only to a population of photoexcited carriers, but also to the generation of coherent electronic states of particular symmetries which can drive vibrations of the same symmetry. For A1g and Eg symmetry phonons in Bi and Sb, there has been some controversy regarding the generation mechanism of coherent vibrations. Here, we use a combination of ultrafast stimulated Raman scattering (RS) and cw spontaneous RS to determine the lifetime of electronic coherences of A1g and Eg symmetry. Their lifetime can be inferred from a comparison between pump-probe measurements of the amplitude of the corresponding coherent phonons, and a determination of the spontaneous RS cross sections. Our results represent a new approach to probe extremely fast electron decoherence rates using much slower (50-100fs FWHM) laser pulses. The Eg electronic coherence, resulting from a fragile unequal distribution of carriers in three equivalent regions of the band structure, is extremely short lived. Its temperature-dependent lifetime is in the range 2-12 fs in Bi and 5-12 fs in Sb. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X22.00005: First-principles calculations of lattice stabilities in Mo Weston Nielson, Vidvuds Ozolins The determination of accurate lattice stabilities is of great importance in producing phase diagrams of metallic alloys using the CALPHAD approach. Ab-initio molecular dynamics simulations in combination with thermodynamic integration are used to determine the lattice stabilities of fcc and bcc phases in Molybdenum at a range of temperatures. We employ the so-called fixed-cell-shape molecular dynamics approach, which involves the calculation of free energies over varying lattice strains. Our results also predict that at high temperatures fcc Mo is harmonically unstable. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X22.00006: Competing orders in the Dirac-like electronic structure and the non-linear sigma model with the topological terms Pouyan Ghaemi, Shinsei Ryu The Dirac-like electronic structure can host a large number of competing orders in the form of mass terms. In particular, two different order parameters, can be said to be dual to each other, when a static defect in one of them traps a quantum number (or ``charge'') of the other. The complementary nature of the pair of the order parameters shows up in their dynamical properties (correlation functions) in the following sense: When a quantum phase transition is driven by one type of fluctuations in order parameter, approaching the transition from the disordered (paramagnetic) side, the order parameter correlation function at the critical point is reduced. On the other hand, such fluctuations enhances the correlation of the dual order parameter. Such complementary behaviors in the correlation function can be used to diagnose the nature of quantum fluctuations that is the driving force of the quantum phase transition. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X22.00007: Concentration dependence of the electron-phonon coupling from metals to semiconductors Andrei Sergeev, Michael Reizer, Vladimir Mitin We study dependence of the deformation potential (DP) on concentration of carriers in the wide range from metals to semiconductors. DP in metals and semiconductors has a different nature. In metals, DP is due to electron gas compressibility, while in semiconductors this contribution is negligible due to small carrier concentrations. DP in semiconductors originates from a shift of the conduction band edge under the deformation, while in metals such contribution is small because of strong screening. We investigate DP in the transition region and found that the electron-phonon coupling has a significant minimum at intermediate concentrations. The effects of disorder on the coupling are also investigated. Theoretical conclusions are compared with available data on semi-metals and highly-doped semiconductors. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X22.00008: Investigation of the electronic structure of TiTe$_{2}$ using ARPES Jianqiao Meng, Gey-Hong Gweon, Andrew LaForge, Sriram Shastry, Arthur Penn Ramirez, Zack Schlesinger, Kai Rossnagel TiTe$_{2}$ is considered a model Fermi liquid material in the field of angle-resolved photoelectron spectroscopy (ARPES). Over the years, many groups have contributed to improving the quality of the ARPES data on TiTe$_{2}$, helping to understand the connection between the ARPES data and the transport properties. However, some key questions remain unanswered, the most outstanding one being the anomalous temperature dependence in the Hall coefficient $R_{H}$. Here, we present a detailed high resolution ARPES data set in a wide range of temperature and momentum. This reveals some new features: temperature dependence in the band width, temperature dependence of the Ti 3d and Te 5p occupancies, and subtle features in the line shapes as the peak crosses the Fermi level. We discuss these new features in comparison with previous ARPES studies and known transport properties. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X22.00009: Anomalous Ordering in Inhomogeneously Strained Materials: Surface Critical Behavior in the Bulk Kevin E. Bassler, Charo I. Del Genio, Bo Li We study a continuous quasi-two-dimensional order-disorder phase transition that occurs in a simple model of a material that is inhomogeneously strained due to the presence of dislocation lines. Performing Monte Carlo simulations of different system sizes and using finite size scaling, we measure critical exponents describing the transition of $\beta =0.18 \pm 0.02$, $\gamma =1.0\pm 0.1$, and $\alpha =0.10\pm 0.02$. Comparable exponents have been reported in a variety of physical systems. These systems undergo a range of different types of phase transitions, including structural transitions, exciton percolation, and magnetic ordering. In particular, similar exponents have been found to describe the development of magnetic order at the onset of the pseudogap transition in hightemperature superconductors. Their common universal critical exponents suggest that the essential physics of the transition in all of these physical systems is the same as in our simple model. We argue that the nature of the transition in our model is related to surface transitions although our model has no free surface. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X22.00010: Finding saddle points using Gentlest ascent dynamics Amit Samanta, Xiang Zhou, Weinan E We present dynamical equations for determining transition states and escape paths from basins of attraction of a stable system on a potential energy landscape. It is shown that the stable fixed points of such dynamical systems are the index-1 saddle points. The method relies on determining the smallest eigenvalue of the Hessian matrix. The formalism is easy to extend to systems of higher dimensions and can be used to explore the free energy landscapes of systems whose large time scale separation makes the standard molecular dynamics inefficient. The utility of the algorithm is demonstrated by evaluating the activation parameters for homogeneous and heterogeneous dislocation nucleation. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X22.00011: An efficient method to treat low barriers in kinetic ART simulations Peter Brommer, Normand Mousseau In kinetic Monte Carlo (KMC) the time scale of the simulation is dominated by the height of the lowest energy barrier separating two states. Rapid back-and-forth movements across very low barriers called flickers are a major limitation of the technique, as they can cost considerable CPU time without advancing the simulation. To accelerate KMC simulations, an energy basin finding algorithm has been presented [1]. In the kinetic Activation-Relaxation technique (kART) [2], KMC events are constructed during the simulation, taking full care of elastic deformations while avoiding the need for complete event search at every step. To account for low energy barriers located in this package, we implement a basin identification scheme that works on the fly as well. We apply this method to interstitial diffusion in bcc iron. There, rapid diffusion paths with low barriers for properly aligned interstitial clusters limit the simulated time. With our method, we can prevent unproductive oscillations in this diffusive basin while maintaining an appropriate distribution of exit states. This considerably extends the time scales accessible to simulation.\newline \noindent [1] Puchala \emph{et al.}, \emph{J.\ Chem.\ Phys.} {\bf 132}, 134104 (2010)\newline [2] El-Mellouhi \emph{et al.}, \emph{Phys.\ Rev.\ B}, {\bf 78}, 153202 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X22.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X22.00013: Is the Debye-temperature a useful concept at the nanometer scale? Insights from\textit{ ab initio} free energy calculations of Au$_{13 }$and Au$_{12}$Fe nanoclusters Ghazal S. Shafai, Marisol Alcantara Ortigoza, Talat S. Rahman We have calculated the phonon density of states, specific heat, and mean-square vibrational amplitudes of the five lowest-lying isomers of Au$_{13}$ and two of Au$_{12}$Fe nanoparticles, as dictated by their Helmholtz free energy. We find the vibrational entropic contributions to not affect the energy ordering of the isomers. We show that the highest phonon frequencies shift to slightly higher energies in the hybrid clusters: a signature of alloying. As expected the density of vibrational states differs significantly from the Debye model for bulk systems. The definition of the ``Debye temperature'' for the nanocrystal thus becomes ambiguous and depends very much on how it is calculated. In particular it neither correlates uniquely with atomic bond strengths nor does it relate to the maximum phonon frequency. The discrete phonon spectrum of nanoparticles is thus needed to describe the \textit{exact} mean square displacement or the temperature dependency of the heat capacity. Work supported by DOE Grant DE-FG02-07ER46354 [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X22.00014: Insights on the anomalously soft and stiff modes of metal nanoparticles Marisol Alcantara Ortigoza, Talat S. Rahman, Rolf Heid, Klaus-Peter Bohnen The low- and high-energy tails of the phonon density of states (PDOS) of transition-metal nanoparticles is enhanced with respect to that of their bulk counterparts. For particles in the sub-nanometer scale, we propose a rationale for this fact based on ab initio calculations of their charge density and of the frequency and displacement pattern of their vibrational modes. We find that the radial breathing and non-radial vibrations -- analogous to the pulsations observed in variable stars -- correspond to the highest and lowest frequencies, respectively. This result is traced to the radial atomic distribution and the charge density distribution particular to the low-coordinated atoms, both of which give rise to modes that have no counterpart in the bulk. We find that the enhanced PDOS at low frequencies is at least partly due to the relatively small number of modes that nanoparticles can sustain and that clusters with bulk-like ordering render fewer and less stiff modes above the bulk limit. [Preview Abstract] |
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