Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U39: Metals: Alloys and Actinide Compounds |
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Sponsoring Units: DCMP Chair: Lin-Lin Wang, Ames Laboratory Room: 348 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U39.00001: Statistical Mechanics of Nanoscale Metallic Materials Based on Thermodynamic Availability Robert Cammarata When characterizing the equilibrium behavior of small metallic systems, capillary effects can strongly influence the thermal behavior and need to be taken into account in a complete thermodynamic analysis. Although a variety of approaches have been offered to incorporate these effects, they sometimes invoke certain intensive thermodynamic quantities (e.g., chemical potentials) that are not well-defined when dealing with a physically and/or chemically inhomogeneous interfacial region.It has been proposed that many of these difficulties can be resolved by employing the thermodynamic availability function rather than the conventional free energy potentials [R.C. Cammarata, Phil Mag. 88, 927 (2008); R.C. Cammarata, Sol. State Phys. 61, 1 (2009)]. When applied to statistical mechanical calculations, capillary effects on nanoscale system behavior can be obtained in a natural and rigorous way. This procedure will be briefly reviewed and then applied to nanoscale metallic fluid and solid systems. Important issues contrasting the thermodynamic differences between fluid and solid surfaces and how they need to be included in order to obtain physically meaningful results will be discussed. Applications to gas adsorption and nucleation will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U39.00002: Grain Rotation and Growth in Nanocrystalline Silver and Silver/Copper Alloys Michael Chandross, Shengfeng Cheng Grain rotation and growth play important roles in nanotribology and the plastic deformation of nanocrystalline metals and alloys. It is difficult, however, to study these processes with full atomistic detail experimentally. We used molecular dynamics simulations to investigate the grain rotation, coalescence, and growth in pure silver and silver/copper alloys after imposing various modes of deformation, including stretch, compression, and shear. Our results show that the degree of grain rotation and growth in pure silver depends on the state of stress in the sample and is most significant under shear deformation, where very large grains are observed after substantial shear. However, in silver/copper alloys, almost no grain growth was found even under strong shear. The presence of atoms with different lattice constants in alloys stabilizes the grain boundaries and makes grain coalescence less energetically favorable. The implications of these results on nanotribology of pure metals and alloys are discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U39.00003: Temperature-driven Phase Transformation in Y$_3$Co: Neutron Scattering and DFT Studies A. Podlesnyak, G. Ehlers, H. Cao, M. Matsuda, M. Frontzek, O. Zaharko, V.A. Kazantsev, A.F. Gubkin, N.V. Baranov The effects of a crystal structure deformation due to subtle atomic displacements have attracted much attention because they can result in colossal changes of the electronic and magnetic properties of solids. The R$_3$Co binary intermetallic systems exhibit a number of complicated phenomena, including field-induced magnetic phase transitions (R=Er, Ho, Tb), giant magnetoresistance (R=Dy), a substantial magnetocaloric effect (R=Gd) and superconductivity (R=La). Contrary to previous studies that defined the ground state crystal structure of the entire R$_3$Co series as orthorhombic Pnma, we find that Y$_3$Co undergoes a structural phase transition upon cooling around Tc~160K. Density functional theory calculations reveal a dynamical instability of the Pnma structure of Y$_3$Co. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\xi)$ acoustic phonon mode below the critical temperature Tc. We suggest that some other members of the R$_3$Co series (or even all of them) have ground state crystal symmetry lower than reported Pnma. This raises a question about the true magnetic structures and hence the influence of magnetic properties of the entire R$_3$Co series. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U39.00004: Optical Absorption Spectrum of Gold from First Principles Jamal Mustafa, Emmanouil Kioupakis, Steven Louie Phonon-assisted optical absorption is an important optical process in metals for photons in the visible part of the spectrum. Developments in first-principles computational methods have enabled the calculation of phonon-mediated optical absorption spectra of materials. The use of Maximally Localized Wannier Functions enables the interpolation of the GW quasiparticle band structure, along with the optical and electron-phonon coupling matrix elements, to very fine meshes in the Brillouin zone, which are needed for the calculation of the phonon-assisted absorption coefficient. We present calculations on gold that include the quasiparticle band structure and lifetimes, phonon dispersion, Wannier functions, and the phonon-assisted absorption spectrum. Since indirect absorption is a second-order process, the lifetime of the virtual intermediate state is of central importance. The results are compared to experimentally determined optical constants. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U39.00005: Less than perfect C$_{\mathrm{2v}}$ symmetry: loss of mirror plane symmetry in angle-resolved photoemission Thomas Scott, Keisuke Fukutani, Hirokazu Hayashi, Tula Paudel, Eike Schwier, Taiki Horike, Yorito Nagata, Jian Jiang, Hideaki Iwasawa, Kenya Shimada, Evgeny Tsymbal, Yaroslav Losovyj, Peter Dowben The effects of lack of in-plane $C_{2}$ invariance of the crystal on the angle-resolved photoemission spectra are investigated for Mo(112). The results indicate that, for Mo(112), the absence of $C_{2}$ symmetry gives rise to noticeable asymmetry in the ARPES band mapping along the \textless\ 1 1 -1\textgreater\ direction. The apparent differences in the experimental band structure in $+$k versus --k wave vectors can be understood quantitatively in terms of the asymmetries in the electronic bulk band structure, photoelectron diffraction as well as the initial state contribution to the photoemission matrix elements. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U39.00006: Prediction of dislocation junction strength in hexagonal close-packed crystals Chi-Chin Wu, Peter Chung Determination of dislocation junction strengths in \textit{hcp} crystals is important in order to understand and control the fundamental mechanisms in plastic deformation in new lightweight metals and to reduce the density of deleterious dislocations in wide band-gap wurtzite semiconductors. The many factors that may be involved, such as combinations of available slip systems, native material properties, and local morphology due to growth conditions, make systematic investigations via combinatorial experimental approaches challenging. Utilizing discrete dislocation (DD) simulations, we determine yield surfaces comprised by loci of critical stresses required to unzip junctions. Then, using a comparative study of different binary junctions formed by non-coplanar dislocations using different pairs of Burgers vectors on different intersecting planes in Mg and Be crystals, we find that the shape and orientation of yield surfaces are most sensitive to the planes on which the junction forms but independent of the elastic properties. The latter only appears to affect the size of yield surface which is consistent with known behavior in fcc crystals. This work particularly detects similarities and differences in dislocation junctions in hcp crystals. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U39.00007: Phonon Engineering in Metals from First Principles Nicholas Lanzillo, J. Thomas, E.B. Watson, M. Washington, Saroj K. Nayak The electron-phonon interaction in metallic systems controls the electronic transport properties, including both electrical and thermal resistivity. The effect of compressive strain on the electron-phonon interaction in metals is investigated using first-principles density functional theory, and we propose various ways to ``engineer'' this interaction for various technological applications. In particular, we show that by applying compressive strain on the FCC crystals of Al, Cu, Ag and Au, the net electron-phonon scattering rate decreases and likewise the electrical resistivity decreases with increasing pressure. This trend is corroborated by experimental measurements of the resistance of a 0.5 mm diameter high-purity Al wire pressurized up to 2 GPa in a solid-media pressure apparatus at room temperature. The rate of the decrease in electrical resistivity as a function of pressure as determined by experiment is matched by the rate predicted by theory. Our simulations show that Al nanowires have the same response to strain as the bulk crystal; the net electron-phonon scattering can be reduced through compressive strain. Modifying the electron-phonon interaction in metallic structures shows great promise for future nano-electronic devices. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U39.00008: Density-functional study of U-TRU-Zr and U-TRU-Mo alloys Alexander Landa, Per Soderlind, Patrice Turchi The U-Zr and U-Mo alloys proved to be very promising fuels for liquid metal fast breeder reactors. The optimal composition of these alloys is determined from the condition that the fuel could remain stable in the bcc phase ($\gamma $-U) in the temperature range of stability of $\alpha $-U phase. In other words, both Zr and Mo play a role of ``$\gamma $-stabilizers'' helping to keep U in the metastable bcc phase upon cooling. In the present study we perform KKR-ASA-CPA and EMTO-CPA calculations of the ground state properties of $\gamma $-U-Zr and $\gamma $-U-Mo alloys and compare their heats of formation with CALPHAD assessments. Though the U-Zr and U-Mo alloys can be used as nuclear fuels, a fast rector operation on a closed fuel cycle will, due to the nuclear reactions, contain significant amount of TRU elements (Np, Pu, and Am). Above mentioned density-functional theory techniques are extended to study ground-state properties of the bcc-based X-Zr and X-Mo (X $=$ Np, Pu, Am) solid solutions. We discuss how the heat of formation correlates with the charge transfer between the alloy components, and how magnetism influences the deviation from Vegard's law for the equilibrium atomic volume. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Work at LLNL was funded by the Laboratory Directed Research and Development Program under project tracking code 12-SI-008. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U39.00009: Thermal properties of UO$_{2}$ single crystal K. Gofryk, S. Du, A.D. Andersson, C.R. Stanek, R. Schulze, D. Safarik, B. Mihaila, J.C. Lashley, J.L. Smith For decades UO$_{2}$ has been the most widely studied actinide oxide because of its technological importance as fuel material for nuclear reactors. Therefore there is a large interest in understanding its thermal, transport and thermodynamic properties. We present recent experimental results for the thermal conductivity and thermal expansion of high quality UO$_{2}$ single crystal, obtained for different crystallographic directions, and compare with results of molecular dynamics simulations. We will discuss the implications of this study. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U39.00010: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U39.00011: Imaging electronic hot spots in the spectral function of the actinide UCoGa$_5$ Matthias J. Graf, Tanmoy Das, Tomasz Durakiewicz, Jian-Xin Zhu, John J. Joyce, John L. Sarrao We performed self-consistent GW-like calculations within the intermediate Coulomb-U coupling regime to investigate dynamic correlation effects in the intermetallic actinide UCoGa$_5$. This material is often used to contrast anomalous behavior in other U-115 and Pu-115 compounds, because it is presumed to be a conventional Fermi liquid that resembles a ``vegetable.'' First-principles electronic structure calculations were used as input, combined with the spin-fluctuation exchange approximation, to compute self-consistently the many-body self-energy responsible for dynamic correlation effects. We validated theory by angle-resolved photoemission spectroscopy (ARPES). The occurrence of electronic hot spots in the spectral function, accompanied by kinks and abrupt breaks in the slope of the quasiparticle dispersion were detected both at low (130 meV) and high (1 eV) binding energies below the Fermi energy. In conclusion, we found that dynamic correlation anomalies are adequately described by coupling between itinerant fermions and spin fluctuations arising from the particle-hole continuum of the spin-orbit-split 5f states of uranium. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U39.00012: Probing the f-state configuration of $\alpha$U and URu$_2$Si$_2$ with RXES Scott Medling, Corwin H. Booth, Ryan Baumbach, Eric D. Bauer We directly probed the electronic configuration of several uranium compounds using Resonant X-ray Emission Spectroscopy (RXES). Previous investigations by several groups into the magnetic properties of uranium compounds (such as URu$_2$Si$_2$) suggested that some are multiconfigurational. RXES is particularly useful for probing the configurations because measuring the energies of both the incident and scattered photons reveals information about both the empty and occupied electronic states. We collected data for several uranium samples ($\alpha$U, UO$_2$, and URu$_2$Si$_2$) which indicate that in some of these compounds the uranium is multiconfigurational, with a mixture of f$^1$,f$^2$, and f$^3$ occupancies. The degree of intermediate valence that this implies will be related to electronic and magnetic properties of the compound. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U39.00013: Towards a Density Functional Theory Exchange-Correlation Functional able to describe localization/delocalization Ann E. Mattsson, John M. Wills The inability to computationally describe the physics governing the properties of actinides and their alloys is the poster child of failure of existing Density Functional Theory exchange-correlation functionals. The intricate competition between localization and delocalization of the electrons, present in these materials, exposes the limitations of functionals only designed to properly describe one or the other situation. We will discuss the manifestation of this competition in real materials and propositions on how to construct a functional able to accurately describe properties of these materials. I addition we will discuss both the importance of using the Dirac equation to describe the relativistic effects in these materials, and the connection to the physics of transition metal oxides. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U39.00014: Actinide electronic structure based on the Dirac equation and density functional theory John M. Wills, Ann E. Mattsson Density functional theory (DFT) provides a formally predictive basis for predicting the structural properties of actinides. Although available approximations to the exchange/correlation functional provide accurate predictions for many materials, they fail qualitatively and sometimes quantitatively when applied to actinides. Major contributors to this deficiency are an inadequate treatment of confinement physics and an incomplete treatment of relativity in the underlying equations. The development of a functional correctly incorporating confinement physics with a proper treatment of relativity would provide definitive, internally consistent predictions of actinide properties. To enable the development of such a functional and quantify the predictions of currently available functionals, we have developed an efficient first-principles electronic structure method based on the Dirac equation. Results are compared with current methods, and the implications for relativistic density functionals discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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