Robert Albers - Academia.edu (original) (raw)

Papers by Robert Albers

J Phys Condens Matter, 2011

The phase stability and site occupancy of bcc (body centered cubic) Nb 5 Al and slightly rearrang... more The phase stability and site occupancy of bcc (body centered cubic) Nb 5 Al and slightly rearranged atomic structures have been examined by means of first-principles calculations. In order to use first-principles methods, a periodic cell is required and we used ordered Nb 5 Al compounds as a tractable example of a low Al concentration Nb 1−x Al x alloy (in this case, for about 17 at.% Al). The instability against an ω-structure atomic displacement was also studied, since this structure is detrimental to ductility. Mulliken population analysis was used to provide an understanding of the hybridization between the atoms and the electronic origin of the site occupancy and instability of the underlying bcc structures. By making calculations for several different configurations of the Nb-Al system we estimated the strengths of the Nb-Nb and Nb-Al bonds. It is shown that the stability of the underlying bcc phases is directly related to Nb-Nb and Nb-Al first-nearest-neighbor interactions. The first-principles calculations were extended to finite temperature by including various contributions to the free energy. In particular, the vibrational free energy was calculated within the quasiharmonic approximation, and it is shown that the contribution of the low energy modes to the lattice entropy helps to stabilize ordered bcc phases against ω-type phase transformations. Semi-quasi-random structures were employed to study the stability of the ordered and disordered bcc phases. Our study showed, in agreement with experiment, that the ω, ordered, and disordered phases can coexist in a nonequilibrium state at finite temperature.

Physical Review B, 2002

A tight-binding model is fit to first-principles calculations for copper that include structures ... more A tight-binding model is fit to first-principles calculations for copper that include structures distorted according to elastic constants and highsymmetry phonon modes. With the resulting model the first-principlesbased phonon dispersion and the free energy are calculated in the quasiharmonic approximation. The resulting thermal expansion, the temperatureand volume-dependence of the elastic constants, the Debye temperature, and the Grüneisen parameter are compared with available experimental data.

Los Ahmos National Laboratoly. an affirmative actiodequal owortunily employer, is operated by the... more Los Ahmos National Laboratoly. an affirmative actiodequal owortunily employer, is operated by the University d Californh for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royaltyfree license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requesls that the publisher Identify this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researchers righ? to publish as an institution. however, the Labomtory does not endorse the vievpint of a publlcatbn or guarantee its technical conedness. Fwm 836 (1-ST 2629 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Physical review. B, Condensed matter, Jan 15, 1985

ABSTRACT The electronic ground states of UAl2, NpAl2, and PuAl2 in the C15 crystal structure were... more ABSTRACT The electronic ground states of UAl2, NpAl2, and PuAl2 in the C15 crystal structure were determined by performing self-consistent, semirelativistic, warped muffin-tin linear augmented-plane-wave electronic band-structure calculations. Then the densities of states and band states were determined by obtaining solutions including spin-orbit interactions with the self-consistent semirelativistic potential. The partial density of states functions obtained for these systems indicate strong d-f hybridization. A consistent procedure for determining the spin fluctuation parameters from the experimental data was obtained. This procedure was used to obtain the spin-fluctuation parameters from UAl2. It is found that this procedure yields parameters that are different from previous estimates and indicates that the calculated parameters such as the bare density of states at the Fermi energy N(0), and the occupied f-band width are consistent with the band-structure values. It is found that although the actinide-actinide separation is about the same, the electronic structure near EF is different. These calculations indicate that both d-f hybridization and spin-orbit coupling are important in the electronic structure of these systems.

A major purpose of the Technical Information Center is to provide the broadest dissemination poss... more A major purpose of the Technical Information Center is to provide the broadest dissemination possible of information contained in DOE'S I?esearch and Development Reports to business, industry, the academic community, and federal, state and local governments. Although a small portion report is not reproducible, made available to expedite being of this it is the availability of information on the research discussed herein.

ABSTRACT The merger of density functional theory in the local density approximation and the many-... more ABSTRACT The merger of density functional theory in the local density approximation and the many-body dynamical mean field theory is a powerful theoretical technique for the study of strongly correlated electron materials. We present calculations of spectral properties of the delta-phase plutonium by combining for the first time the sophisticated tight-binding method with a recent implementation of quantum Monte Carlo techinque. The tight-binding parameters are determined from the fit to the full-potential linearized augmented plane-wave calculation for the face-centered-cubic crystal structure of the slightly compressed delta-phase plutonium. The computationally more expensive but rigorous quantum Monte Carlo simulation is supplemented by the more efficient but approximate Hubbard-I method. By comparing the calculations without and with spin-orbit interaction included, we discuss our results in the context of several key features observed in the photoemission spectroscopy.

This is the final report of a two-year, Laboratory-Directed Research and Development (LDRD) progr... more This is the final report of a two-year, Laboratory-Directed Research and Development (LDRD) program development project at the Los Alamos National Laboratory (LANL). The purpose of this project was to develop new capabilities to assess the nucleation and growth of helium associated defects in aged plutonium metal. This effort involved both fundmental and applied models to assist in predicting the transport and kinetics of helium in the metal lattice as well as ab initio calculations of the disposition of Ga in the fcc plutonium lattice, and its resulting effects on phase stability. Experimentally, this project aimed to establish experimental capabilities crutial to the prediction of helium effects in metals, such as transmission electron microscopy, thermal helium effusion, and the development of laser driven mini-flyer for understanding the role of helium and associated defects on shock response of plutonium surrogates. Background and Research Objectives The aging of both active and inactive weapons in the US nuclear arsenal is an acknowledged issue of great importance for the Stockpile Stewardship and Management Program of the DOE. Detailed knowledge of the mechanisms of aging and the consequences on component performance are important not only for predicting changes in overall physics package reliability, but are perhaps most important for proper scheduling of component change out and sizing of the appropriate facilities for such refurbishments and replacement activities. In addition, detailed knowledge of stockpile materials, such as plutonium, is now needed to establish recertification and replacement standards. As new

We propose a new, alternative method for ab-initio calculations of the electronic structure of so... more We propose a new, alternative method for ab-initio calculations of the electronic structure of solids, which has been specifically adapted to treat many-body effects in a more rigorous way than many existing ab-initio methods. We start from a standard band-structure calculation for an effective one-particle Hamiltonian approximately describing the material of interest. This yields a suitable set of one-particle basis functions, from which well localized Wannier functions can be constructed using a method proposed by Marzari and Vanderbilt. Within this (minimal) basis of localized Wannier functions the matrix elements of the non-interacting (one-particle) Hamiltonian as well as the Coulomb matrix elements can be calculated. The result is a many-body Hamiltonian in second quantization with parameters determined from first principles calculations for the material of interest. The Hamiltonian is in the form of a multi-band Hamiltonian in second quantization (a kind of extended, multi-ba...

Physical Review Letters, 2007

Physical Review B, 2007

By combining the local density approximation (LDA) with dynamical mean field theory (DMFT), we re... more By combining the local density approximation (LDA) with dynamical mean field theory (DMFT), we report a systematic analysis of the spectral properties of δ-plutonium with varying 5f occupancy. The LDA Hamiltonian is extracted from a tight-binding (TB) fit to full-potential linearized augmented plane-wave (FP-LAPW) calculations. The DMFT equations are solved by the exact quantum Monte Carlo (QMC) method and by the Hubbard-I approximation. We demonstrate strong sensitivity of the spectral properties to the 5f occupancy, which suggests using this occupancy as a fitting parameter in addition to the Hubbard U. By comparing with photoemission data, we conclude that the "open shell" 5f 5 configuration gives the best agreement, resolving the controversy over 5f "open shell" versus "close shell" atomic configurations in δ-Pu.

Journal of Ministry in Addiction & Recovery, 1998

Journal of Low Temperature Physics, 1979

The memory function formalism is used to derive a generalized golden rule expression for the spin... more The memory function formalism is used to derive a generalized golden rule expression for the spin-lattice relaxation rate l/T1 for 3He to paramagnetic centers embedded in or residing on surfaces in contact with the 3He. This expression is applied to several simple models of relaxation to paramagnetic 3 spins which do not in teract among themselves, both for He in the Fermi-liquid phase and for a solid surface layer of 3He on the substrate. The magnitude of 1/T1 is calculated for each case as well as the temperature and magnetic field dependence. Finally, the relationship between the spin-lattice relaxation rate and the Kapitza conductance across surfaces via magnetic interactions is determined.

Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si ha... more Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si have been studied using Stillinger-Weber and Tersoff classical potentials, and semi-empirical (tight-binding) electronic structure methods. The calculated lowest energy (310) grain boundary structure and electronic properties are consistent with previous TEM measurement and calculations. For the controversial (710) grain boundaries, the tight-binding calculations do not show any electronic energy levels in the band gap. This indicates that with every atom fully fourfold coordinated, the (710) grain boundary should be electrically inactive. Some high-energy metastable grain boundaries were found to be electrically active by the presence of the levels introduced in the band gap. Also, the vacancy concentration at the (310) GB was found to be enhanced by many orders of magnitude relative to bulk. The dangling bond states of the vacancies should be electrically active.

Journal of Applied Physics, 1991

ABSTRACT