Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V28: Actinides, ... |
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Chair: Robert Albers, LANL Room: LACC 506 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V28.00001: Strong electron-phonon coupling in $\delta$-phase stabilized Pu Matthias J. Graf, Turab Lookman, John M. Wills, Duane C. Wallace, Jason C. Lashley Specific heat measurements of the $\delta$-phase stabilized alloy Pu$_{0.95}$Al$_{0.05}$ suggest that strong electron-phonon coupling is required to explain the moderate renormalization of the electronic density of states near the Fermi energy. We calculated the specific heat contributions from the lattice and electronic degrees of freedom as well as from the electron-lattice coupling term and find good agreement between experiment and theory assuming a dimensionless electron-phonon coupling parameter of order unity, $\lambda \sim 0.8$. This large electron-phonon coupling parameter is comparable to reported values in other metals with face-centered cubic crystal structure, for example, Pd ($\lambda \approx 0.7$) and Pb ($\lambda \approx 1.5$). Further our analysis shows evidence of a small residual entropy contribution, $S_{res} \approx 0.4 \, k_B$ (per atom), which is consistent with an earlier proposed scenario that with decreasing temperature parts of the sample undergo a structural phase transformation. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V28.00002: Point defects in alpha and delta plutonium Babak Sadigh, Wilhelm Wolfer We present first-principles calculations of the energetics of vacancies and interstitials in alpha and delta Pu. We perform calculations within spin-polarized density-functional theory. We analyze the lattice distortions around the point defects in terms of elastic relaxations as well as distortions induced by the change in the electronic structure of the host atoms due to the addition of the defect. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V28.00003: A Density Functional Study of Molecular Oxygen Adsorption and Reaction Barrier on Pu (100) Surface M.N. Huda, A.K. Ray Oxygen molecule adsorptions on a Pu (100) surface have been studied in detail, using the generalized gradient approximation to density functional theory and the DMol3 suite of programs [1]. Dissociative adsorption with a layer by layer alternate spin arrangement of the plutonium layer is found to be energetically more favorable compared to molecular adsorption. \textit{Hor}2 approach on a bridge site without spin polarization was found to the highest chemisorbed site with an energy of 8.787eV among all the cases studied. The second highest chemisorption energy of 8.236eV, is the spin-polarized \textit{Hor2 }or \textit{Ver} approach at center site. Inclusion of spin polarization affects the chemisorption processes significantly, non-spin-polarized chemisorption energies being typically higher. We also find that the 5f electrons to be more localized in spin-polarized cases. The ionic part of O-Pu bonding plays a significant role, while the Pu 5f-O 2p hybridization is rather weak. Adsorptions of oxygen push the top of 5f band away from the Fermi level, indicating further bonding by the 5f orbitals might be less probable. [1] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); J. Chem. Phys. \textbf{113}, 7756 (2000). [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V28.00004: Equilibrium magnetic structure of NpSn$_3$ M. Taghi Fallahi, Zahra Nourbakhsh Actinide based compounds show very intersting physical properties due to the singular behaviour of their 5f electrons. The variation of these properties yields information on the electronic structure of the solid. We have focused on the NpSn$_{3 }$which exhibits antiferomagnetic phase at zero pressure. In this work to investigate the magnetic properties of NpSn$_{3}$, the total energy per primitive unit cell as a function of its volume in antiferomagnetic, feromagnetic and nonmagnetic phases are calculated using density functional theory within GGA and LDA approaches. We have found an antiferomagnetic ground state for NpSn$_{3}$ in both approches. We have also found that for unit cell valumes smaller than a certain value, the magnetic moment is quenched and therfore the two antiferomagnetic and nonmagnetic curves coincide. For larger volumes a net magnetic moment appears and the antiferomagnetic curve branches off the nonmagnetic one. The branching valoumes found in different approaches are different. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V28.00005: Poisson’s ratio: key to electron localization in plutonium Albert Migliori, Hassel Ledbetter Focusing on plutonium, we show that a familiar usually-ignored elastic parameter---the dimensionless Poisson ratio ($\nu )$---helps understand a knotty unsolved problem: itinerant-localized 5f electrons. A simple electrostatic electron-gas model predicts that the bulk modulus $B$ and shear modulus $G$ decrease as electrons change from itinerant to localized (the free-electron-gas density decreases), but that the Poisson ratio remains unchanged. From the bulk modulus, the model predicts approximate divalency for plutonium, implying a 5$f^{4}$ localized-electron configuration. We deduce that warming plutonium from 0 to 300 K causes 0.24 electrons/atom to change from itinerant state to localized. Sufficient alloying with Ga or Al suppresses the itinerant---localized transition, causing the Poisson ratio to show near-typical temperature dependence. This work was carried out at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation, the State of Florida and DOE. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V28.00006: Photoemission Spectroscopy on Single Crystal Uranium (001) C.P. Opeil, R.K. Schulze, R.C. Albers, B. Mihaila, K.B. Blagoev, M.E. Manley, J.C. Lashley, J.L. Smith, P.B. Littlewood We have conducted XPS and UPS measurements (173\underline { $<$} T\underline { $<$} 873 K) on high quality single-crystal U(001) and compared them with WIEN2K code electronic band structure calculations. We report significant correspondence between theory and experiment for binding energy (BE) $<$ 4 eV and report anomalous splitting of the 6p$_{1/2}$, 6p$_{3/2 }$bands at BE = 13-30 eV. Spectrometer resolution allows observation of a shift in spectral weight in the U6d-U5f electrons upon comparison of He I (21.21 eV) and He II (40.80 eV) spectra at T = 173 K. ARPES experiments confirm that our electron structure result is a representative bulk effect. LEED patterns indicate a well-ordered orthorhombic crystallographic surface structure. Additionally, our experiments show a temperature correlation between the U work function and the brittle to ductile transition at $\sim $ 450 K. Recent resistivity and specific-heat data will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V28.00007: The effect of pressure on the electric field gradient in CeAgSb$_2$ Azam Pourghazi, Zahra Nourbakhsh CeAgSb$_{2}$ is a rare example of a ferromagnetically ordered heavy-fermion compound. It presents an opportunity to investigate the physical behavior of a system near a ferromagnetic quantum-critical point. In the vicinity of the quantum critical point, a non-Fermi-liquid character and/or unconventional superconductivity has been found by several workers. In this work, to obtain direct information about the ground-state electronic structure of CeAgSb$_{2}$ the total energy per primitive unit cell as a function of its volume is calculated, using density functional approach within GGA. We have found a ferromagnetic ground state for CeAgSb$_{2}$ at zero pressure. Furthermore, we have calculated the electric field gradient (EFG), which is a direct measure of anisotropy in the charge distribution throughout the crystal. We have found that in this 4f metallic compound, EFG increases with pressure. Similar behavior has been reported for sp and 5f metals. We have also found that the value of EFG at Ag and Ce sites are smaller than its value at Sb(I) and Sb(II) sites. The EFG is larger at Sb(I) than it is at Sb(II) site as expected due to the anisotropy of the electric field at Sb(I) position. The sign of EFG's are positive at Ce and Sb(II) sites and negative at other sites. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V28.00008: Magnetocaloric Effect in GdAl2 and GdNi2 Intermetallics Compounds: A Monte Carlo Study Amos Troper, Eduardo N\'obrega, Pedro Von Ranke, Nilson Oliveira The magnetocaloric effect (MCE) is intrinsic to all magnetic materials and is due to the coupling of crystalline and magnetic lattices with the external magnetic field. The search for new magnetic materials with giant magnetocaloric effect has been challenging physicists in the last decades. Despite the theoretical works reported in the literature, many aspects of the physical processes involved in the MCE are not yet well understood. The theoretical microscopic description of this effect in pure rare earth compounds is usually made by using the Heisenberg model Hamiltonian, treated in the molecular field approximation. However, if we want to study the MCE in rare earth disordered compounds, we should go beyond the molecular field approximation in order to correctly account for the magnetic interactions present in these systems. In this work we study the MCE in rare earth Laves phase binaries e. g., GdAl$_{2}$ and GdNi$_{2}$. In order to describe the magnetic properties of these compounds, we use a classical Heisenberg-like model including disorder in the magnetic lattice. The magnetic and thermodynamic properties are calculated via the Numerical Monte Carlo simulation. The isothermal magnetic entropy changes, calculated using the Maxwell thermodynamic relation are in good agreement with the available experimental data. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V28.00009: Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys Alexey Zayak, Karin Rabe, Peter Entel First-principles calculations are used to address the problem of phonon anomalies in non-magnetic and magnetic Heusler alloys. Phonon dispersions of the cubic L2$\mathrm{_1}$ structure were obtained along the [110] direction. We consider compounds that exhibit phonon instabilities and compare them with their stable counterparts. The analysis of the electronic structure allows us to identify the characteristic features leading to structural instabilities. The phonon dispersions of the unstable structures show that, while the acoustic modes tend to soften, the optical modes disperse in a way that is significantly different from that of the stable structures. In an external field the affected optical modes are Raman active, which is considered as an indication of a stronger covalent interaction in the case of unstable systems. This covalent interaction changes the usual metallic behaviour of some Heusler alloys leading to the possibility of localization of vibrational modes and a possible relationship to similar features observed in perovskites. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V28.00010: Towards a statistical theory of transport by strongly-interacting lattice fermions Subroto Mukerjee, Vadim Oganesyan, David Huse We present a study of electrical transport at high temperature in a model of strongly interacting spinless fermions without disorder. We use exact diagonalization to study the statistics of the energy eigenvalues, eigenstates, and the matrix elements of the current.These suggest that our Hamiltonian is a member of a certain ensemble of Gaussian random matrices. We calculate the conductivity $\sigma(\omega)$ and examine its behavior, both in finite size samples and as extrapolated to the thermodynamic limit. We find that $\sigma(\omega)$ has a prominent non-divergent singularity at $\omega=0$ reflecting a power-law long-time tail in the current autocorrelation function that arises from nonlinear couplings between the long- wavelength diffusive modes of the energy and particle number. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V28.00011: How a new paradigm for quasi one dimensional transport problems overlaps with Schwinger's electron-positron nucleation results as generalized by Lin Andrew Beckwith I present how to apply a wavefunctional formulation of tunneling Hamiltonians to a driven sine~Gordon system. Specifically, I apply a generalization of the tunneling Hamiltonian to charge density wave (CDW) transport problems in which I consider tunneling between states that are wavefunctionals of a scalar quantum field $\phi $. I present previously derived \textbf{\textit{I-E}} curves that match Zenier curves used to fit data experimentally with wavefunctionals congruent with the false vacuum hypothesis. Additionally, I observe a very strong convergence with a generalization of Lin's expansion of Schwinger's work in electron- positron pair production, finding that the similarities argue in favor of the new pinning gap paradigm proposed for quasi-one-dimensional metallic transport problems. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V28.00012: Local geometry of Fermi surfaces and spectra of cyclotron waves in metals Grigory M. Zimbovsky, Natalya A. Zimbovskaya It is well known that transverse modes could propagate in metals along the external magnetic fields, whose frequencies at small wave vectors are close to the electron cyclotron frequency. These collective exitations occur due to the electron-electron interactions described within the framework of the Fermi-liquid theory. Here, we show that when the Fermi surface of a metal includes nearly parabolical segments, this strongly affects spectra of the Fermi-liquid cyclotron waves. Both frequency and wave vector ranges of these modes become significantly broadened, and the dispersion curves could appear extended into the low frequency region. The effect arises due to the singularity in the dielectric function at the boundary of the transparency region which could be strengthened for a certain Fermi surface geometry. Possible manifestations of the effect in experiments are discussed. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V28.00013: \textit{Ab Initio} Theory of Nuclear Magnetic Resonance Shifts in Metals Mayeul d'Avezac, Nicola Marzari, Francesco Mauri A comprehensive approach for the first-principles determination of all-electron NMR shifts in metallic systems is presented. Our formulation is based on a combination of density-functional perturbation theory and all-electron wavefunction reconstruction, starting from periodic-boundary calculations in the pseudopotential approximation. The orbital contribution to the NMR shift (the chemical shift) is obtained by combining the gauge-including projector augmented-wave approach (GIPAW), originally developed for the case of insulators\footnote{C.~J.~Pickard, Francesco Mauri, Phys.~Rev.~B, {\bf 63}, 245101(2001)}, with the extension of linear-response theory to the case of metallic systems\footnote{S.~{de Gironcoli}, Phys.~Rev.~B, {\bf 51}, 6773(1995)}. The spin contribution (the Knight shift) is obtained as a response to a finite uniform magnetic field, and through reconstructing the hyperfine interaction between the electron-spin density and the nuclear spins with the projector augmented-wave method (PAW\footnote{C.~G.~{Van de Walle}, P.~E.~Bl\"ochl, Phys.~Rev.~B, {\bf 47}, 4244(1993)}). Our method is validated with applications to the case of the homogeneous electron gas and of simple metals. (Work supported by MURI grant DAAD 19-03-1-0169 and MIT-France) [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V28.00014: Unified Theory of Field Emission and Thermionic Emission Heetae Kim, Isaac Silvera There are two well-known kinds of electron emissions from metal surfaces: field emission and thermionic emission. For thermionic emission electrons are emitted when they have sufficient thermal energy to overcome the work function and for field emission,as Fowler-Nordheim tunneling, they are emitted by tunneling, aided by a strong repulsive electric field.Ê We considered the most general case of both process together, using a free electron gas model.Ê This can be important in the case of a field emission current ohmically heating the emission tip. The total emission current density is compared to field emission for different temperatures and to thermionic emission for different electric fields. [Preview Abstract] |
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V28.00015: An open question: Are topological arguments helpful in setting initial conditions for transport problems in condensed matter physics? Andrew Beckwith The tunneling Hamiltonian is a proven method to treat particle tunneling between different states represented as wavefunctions in many-body physics. Our problem is how to apply a wave functional formulation of tunneling Hamiltonians to a driven sine-Gordon system. We apply a generalization of the tunneling Hamiltonian to charge density wave (CDW) transport problems in which we consider tunneling between states that are wavefunctionals of a scalar quantum field $\phi $. We present derived \textbf{\textit{I-E}} curves that match Zenier curves used to fit data experimentally with wavefunctionals congruent with the false vacuum hypothesis. The open question is whether the coefficients picked in both the wavefunctionals and the magnitude of the coefficients of the driven sine Gordon physical system should be picked by topological charge arguments that in principle appear to assign values that have a tie in with the false vacuum hypothesis first presented by Sidney Coleman. [Preview Abstract] |
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