127 research outputs found
Expansion algorithm for the density matrix
A purification algorithm for expanding the single-particle density matrix in
terms of the Hamiltonian operator is proposed. The scheme works with a
predefined occupation and requires less than half the number of matrix-matrix
multiplications compared to existing methods at low (90%)
occupancy. The expansion can be used with a fixed chemical potential in which
case it is an asymmetric generalization of and a substantial improvement over
grand canonical McWeeny purification. It is shown that the computational
complexity, measured as number of matrix multiplications, essentially is
independent of system size even for metallic materials with a vanishing band
gap.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Isolated oxygen defects in 3C- and 4H-SiC: A theoretical study
Ab initio calculations in the local-density approximation have been carried out in SiC to determine the possible configurations of the isolated oxygen impurity. Equilibrium geometry and occupation levels were calculated. Substitutional oxygen in 3C-SiC is a relatively shallow effective mass like double donor on the carbon site (O-C) and a hyperdeep double donor on the Si site (O-Si). In 4H-SiC O-C is still a double donor but with a more localized electron state. In 3C-SiC O-C is substantially more stable under any condition than O-Si or interstitial oxygen (O-i). In 4H-SiC O-C is also the most stable one except for heavy n-type doping. We propose that O-C is at the core of the electrically active oxygen-related defect family found by deep level transient spectroscopy in 4H-SiC. The consequences of the site preference of oxygen on the SiC/SiO2 interface are discussed
Maximally-localized Wannier functions for entangled energy bands
We present a method for obtaining well-localized Wannier-like functions (WFs)
for energy bands that are attached to or mixed with other bands. The present
scheme removes the limitation of the usual maximally-localized WFs method (N.
Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of
interest should form an isolated group, separated by gaps from higher and lower
bands everywhere in the Brillouin zone. An energy window encompassing N bands
of interest is specified by the user, and the algorithm then proceeds to
disentangle these from the remaining bands inside the window by filtering out
an optimally connected N-dimensional subspace. This is achieved by minimizing a
functional that measures the subspace dispersion across the Brillouin zone. The
maximally-localized WFs for the optimal subspace are then obtained via the
algorithm of Marzari and Vanderbilt. The method, which functions as a
postprocessing step using the output of conventional electronic-structure
codes, is applied to the s and d bands of copper, and to the valence and
low-lying conduction bands of silicon. For the low-lying nearly-free-electron
bands of copper we find WFs which are centered at the tetrahedral interstitial
sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macro
Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium
We have investigated the structural sequence of the high-pressure phases of
silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase
transitions. We have used the plane-wave pseudopotential approach to the
density-functional theory implemented within the Vienna ab-initio simulation
package (VASP). We have determined the equilibrium properties of each structure
and the values of the critical parameters including a hysteresis effect at the
phase transitions. The order of the phase transitions has been obtained
alternatively from the pressure dependence of the enthalpy and of the internal
structure parameters. The commonly used tangent construction is shown to be
very unreliable. Our calculations identify a first-order phase transition from
the cd to the beta-tin and from the Imma to the sh phase, and they indicate the
possibility of a second-order phase-transition from the beta-tin to the Imma
phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
Stability, Electronic Structure and Vibrational Modes of Ti_8C_12 Dimer
We present our density functional results of the geometry, electronic
structure and dissociation energy of Ti_8C_12 dimer. We show that as opposed to
the currently held view that Ti_8C_12 are highly stable monodispersed clusters,
the neutral Ti_8C_12 clusters form covalent bonds and form stable dimers. We
determine that the Ti atoms bond weakly (0.9 eV/bond) to organic ligands such
as ammonia. Alternatively the Met-Car dimer has a cohesive energy of 4.84 eV or
approximately 1.2 eV per bond. While Met-Car dimers are stable, formation of
these dimers may be quenched in an environment that contains a significant
population of organic ligands. The ionization and dissociation energies of the
dimer are of same order which prevents the observation of the dimer in the ion
mass spectroscopy. The analysis of the vibrational frequencies show the
lowest-energy structure to be dynamically stable. We also present infrared
absorption and Raman scattering spectra of the Ti_8C_12 dimer.Comment: 5 pages, 3 figures (Better quality figures available on request).
Physical Review B (Rapid Communication) (2002, in press
Whole genome analysis of a schistosomiasis-transmitting freshwater snail
Biomphalaria snails are instrumental in transmission of the human blood fluke Schistosoma mansoni. With the World Health Organization's goal to eliminate schistosomiasis as a global health problem by 2025, there is now renewed emphasis on snail control. Here, we characterize the genome of Biomphalaria glabrata, a lophotrochozoan protostome, and provide timely and important information on snail biology. We describe aspects of phero-perception, stress responses, immune function and regulation of gene expression that support the persistence of B. glabrata in the field and may define this species as a suitable snail host for S. mansoni. We identify several potential targets for developing novel control measures aimed at reducing snail-mediated transmission of schistosomiasis
Organic Superconductors: when correlations and magnetism walk in
This survey provides a brief account for the start of organic
superconductivity motivated by the quest for high Tc superconductors and its
development since the eighties'. Besides superconductivity found in 1D organics
in 1980, progresses in this field of research have contributed to better
understand the physics of low dimensional conductors highlighted by the wealth
of new remarkable properties. Correlations conspire to govern the low
temperature properties of the metallic phase. The contribution of
antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the
theory is borne out by experimental investigations and supports
supercondutivity emerging from a non Fermi liquid background. Quasi one
dimensional organic superconductors can therefore be considered as simple
prototype systems for the more complex high Tc materials.Comment: 41 pages, 21 figures to be published in Journal of Superconductivity
and Novel Magnetis
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