306 research outputs found
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Large-scale quantum mechanical simulations of high-Z metals
High-Z metals constitute a particular challenge for large-scale ab initio calculations, as they require high resolution due to the presence of strongly localized states and require many eigenstates to be computed due to the large number of electrons and need to accurately resolve the Fermi surface. Here, we report recent findings on high-Z materials, using an efficient massively parallel planewave implementation on some of the largest computational architectures currently available. We discuss the particular architectures employed and methodological advances required to harness them effectively. We present a pair-correlation function for U, calculated using quantum molecular dynamics, and discuss relaxations of Pu atoms in the vicinity of defects in aged and alloyed Pu. We find that the self-irradiation associated with aging has a negligible effect on the compressibility of Pu relative to other factors such as alloying
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AB Initio Calculations of Elastic Constants of BCC V-NB System at High Pressures
First-principles total energy calculation based on the exact muffin-tin orbital and full potential linear muffin-tin orbital methods were used to calculate the equation of state and shear elastic constants of bcc V, Nb, and the V{sub 95}Nb{sub 05} disordered alloy as a function of pressure up to 6 Mbar. We found a mechanical instability in C{sub 44} and a corresponding softening in C at pressures {approx} 2 Mbar for V. Both shear elastic constants show softening at pressures {approx} 0.5 Mbar for Nb. Substitution of 5 at. % of V with Nb removes the instability of V with respect to trigonal distortions in the vicinity of 2 Mbar pressure, but still leaves the softening of C{sub 44} in this pressure region. We argue that the pressure induced shear instability (softening) of V (Nb) originates from the electronic system and can be explained by a combination of the Fermi surface nesting, electronic topological transition, and band Jahn-Teller effect
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Quantum-Based Atomistic Simulation of Metals at Extreme Conditions
First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for bridging the quantum-atomistic gap from density-functional quantum mechanics to large scale atomistic simulation in metals and alloys. In directionally-bonded bcc transition metals, advanced generation model GPT or MGPT potentials based on canonical d bands have been developed for Ta, Mo and V and successfully applied to a wide range of thermodynamic and mechanical properties at both ambient and extreme conditions of pressure and temperature, including high-pressure phase transitions, multiphase equation of state; melting and solidification; thermoelasticity; and the atomistic simulation of point defects, dislocations and grain boundaries needed for the multiscale modeling of plasticity and strength. Recent algorithm improvements have also allowed an MGPT implementation beyond canonical bands to achieve increased accuracy, extension to f-electron actinide metals, and high computational speed. A further advance in progress is the development temperature-dependent MGPT potentials that subsume electron-thermal contributions to high-temperature properties
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Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots
Recent theoretical descriptions as to the magnitude of effect that quantum confinement has on he conduction band (CB) of CdSe quantum dots (QD) have been conflicting. In this manuscript, we experimentally identify quantum confinement effects in the CB of CdSe QDs for the first time. Using X-ray absorption spectroscopy, we have unambiguously witnessed the CB minimum shift to higher energy with decreasing particle size and have been able to compare these results to recent theories. Our experiments have been able to identify which theories correctly describe the CB states in CdSe QDs. In particular, our experiments suggest that multiple theories describe the shifts in the CB of CdSe QDs and are not mutually exclusive
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
First principles elastic constants and electronic structure of alpha-Pt_2Si and PtSi
We have carried out a first principles study of the elastic properties and
electronic structure for two room-temperature stable Pt silicide phases,
tetragonal alpha-Pt_2Si and orthorhombic PtSi. We have calculated all of the
equilibrium structural parameters for both phases: the a and c lattice
constants for alpha-Pt_2Si and the a, b, and c lattice constants and four
internal structural parameters for PtSi. These results agree closely with
experimental data. We have also calculated the zero-pressure elastic constants,
confirming prior results for pure Pt and Si and predicting values for the six
(nine) independent, non-zero elastic constants of alpha-Pt_2Si (PtSi). These
calculations include a full treatment of all relevant internal displacements
induced by the elastic strains, including an explicit determination of the
dimensionless internal displacement parameters for the three strains in
alpha-Pt_2Si for which they are non-zero. We have analyzed the trends in the
calculated elastic constants, both within a given material as well as between
the two silicides and the pure Pt and Si phases. The calculated electronic
structure confirms that the two silicides are poor metals with a low density of
states at the Fermi level, and consequently we expect that the Drude component
of the optical absorption will be much smaller than in good metals such as pure
Pt. This observation, combined with the topology found in the first principles
spin-orbit split band structure, suggests that it may be important to include
the interband contribution to the optical absorption, even in the infrared
region.Comment: v1: 27 pages, 7 figures, 13 tables submitted to Phys. Rev. B v2: 10
pages, 4 figures, 12 tables (published in Phys. Rev B) contains only
ab-initio calculations; valence force field models are now in a separate
paper: cond-mat/010618
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