427 research outputs found
LDA+DMFT computation of the electronic spectrum of NiO
The electronic spectrum, energy gap and local magnetic moment of paramagnetic
NiO are computed by using the local density approximation plus dynamical
mean-field theory (LDA+DMFT). To this end the noninteracting Hamiltonian
obtained within the local density approximation (LDA) is expressed in Wannier
functions basis, with only the five anti-bonding bands with mainly Ni 3d
character taken into account. Complementing it by local Coulomb interactions
one arrives at a material-specific many-body Hamiltonian which is solved by
DMFT together with quantum Monte-Carlo (QMC) simulations. The large insulating
gap in NiO is found to be a result of the strong electronic correlations in the
paramagnetic state. In the vicinity of the gap region, the shape of the
electronic spectrum calculated in this way is in good agreement with the
experimental x-ray-photoemission and bremsstrahlung-isochromat-spectroscopy
results of Sawatzky and Allen. The value of the local magnetic moment computed
in the paramagnetic phase (PM) agrees well with that measured in the
antiferromagnetic (AFM) phase. Our results for the electronic spectrum and the
local magnetic moment in the PM phase are in accordance with the experimental
finding that AFM long-range order has no significant influence on the
electronic structure of NiO.Comment: 15 pages, 6 figures, 1 table; published versio
Soliton-dynamical approach to a noisy Ginzburg-Landau model
We present a dynamical description and analysis of non-equilibrium
transitions in the noisy Ginzburg-Landau equation based on a canonical phase
space formulation. The transition pathways are characterized by nucleation and
subsequent propagation of domain walls or solitons. We also evaluate the
Arrhenius factor in terms of an associated action and find good agreement with
recent numerical optimization studies.Comment: 4 pages (revtex4), 3 figures (eps
Light emission from silicon with tin-containing nanocrystals
Tin-containing nanocrystals, embedded in silicon, have been fabricated by
growing an epitaxial layer of Si_{1-x-y}Sn_{x}C_{y}, where x = 1.6 % and y =
0.04 %, followed by annealing at various temperatures ranging from 650 to 900
degrees C. The nanocrystal density and average diameters are determined by
scanning transmission-electron microscopy to ~ 10^{17} cm^{-3} and ~ 5 nm,
respectively. Photoluminescence spectroscopy demonstrates that the light
emission is very pronounced for samples annealed at 725 degrees C, and
Rutherford back-scattering spectrometry shows that the nanocrystals are
predominantly in the diamond-structured phase at this particular annealing
temperature. The origin of the light emission is discussed.Comment: 5 pages, 3 figures, submitted to AIP Advance
Transverse Momentum Spectra in Au+Au and d+Au Collisions at =200 GeV and the Pseudorapidity Dependence of High p Suppression
We present spectra of charged hadrons from Au+Au and d+Au collisions at
GeV measured with the BRAHMS experiment at RHIC. The
spectra for different collision centralities are compared to spectra from collisions at the same energy scaled by the number of binary
collisions. The resulting ratios (nuclear modification factors) for central
Au+Au collisions at and evidence a strong suppression in
the high region (2 GeV/c). In contrast, the d+Au nuclear
modification factor (at ) exhibits an enhancement of the high
yields. These measurements indicate a high energy loss of the high
particles in the medium created in the central Au+Au collisions. The lack of
suppression in d+Au collisions makes it unlikely that initial state effects can
explain the suppression in the central Au+Au collisions.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Domain wall propagation and nucleation in a metastable two-level system
We present a dynamical description and analysis of non-equilibrium
transitions in the noisy one-dimensional Ginzburg-Landau equation for an
extensive system based on a weak noise canonical phase space formulation of the
Freidlin-Wentzel or Martin-Siggia-Rose methods. We derive propagating nonlinear
domain wall or soliton solutions of the resulting canonical field equations
with superimposed diffusive modes. The transition pathways are characterized by
the nucleations and subsequent propagation of domain walls. We discuss the
general switching scenario in terms of a dilute gas of propagating domain walls
and evaluate the Arrhenius factor in terms of the associated action. We find
excellent agreement with recent numerical optimization studies.Comment: 28 pages, 16 figures, revtex styl
The Cerium volume collapse: Results from the LDA+DMFT approach
The merger of density-functional theory in the local density approximation
(LDA) and many-body dynamical mean field theory (DMFT) allows for an ab initio
calculation of Ce including the inherent 4f electronic correlations. We solve
the DMFT equations by the quantum Monte Carlo (QMC) technique and calculate the
Ce energy, spectrum, and double occupancy as a function of volume. At low
temperatures, the correlation energy exhibits an anomalous region of negative
curvature which drives the system towards a thermodynamic instability, i.e.,
the -to- volume collapse, consistent with experiment. The
connection of the energetic with the spectral evolution shows that the physical
origin of the energy anomaly and, thus, the volume collapse is the appearance
of a quasiparticle resonance in the 4f-spectrum which is accompanied by a rapid
growth in the double occupancy.Comment: 4 pages, 3 figure
Unquenched large orbital magnetic moment in NiO
Magnetic properties of NiO are investigated by incorporating the spin-orbit
interaction in the LSDA+U scheme. It is found that the large part of orbital
moment remains unquenched in NiO. The orbital moment contributes about mu_L =
0.29 mu_B to the total magnetic moment of M = 1.93 mu_B, as leads to the
orbital-to-spin angular momentum ratio of L/S = 0.36. The theoretical values
are in good agreement with recent magnetic X-ray scattering measurements.Comment: 4 pages, 2 figure
Modification of the standard model for the lanthanides
We show that incorporation of strong electron correlations into the Kohn-Sham
scheme of band structure calculations leads to a modification of the standard
model of the lanthanides and that this procedure removes the existing
discrepancy between theory and experiment concerning the ground state
properties. Within the picture suggested, part of the upper Hubbard -band is
occupied due to conduction band--mixing interaction (that is renormalized
due to correlations) and this contributes to the cohesive energy of the
crystal. The lower Hubbard band has zero width and describes fermionic
excitations in the shell of localized -s. Fully self-consistent calculations
(with respect to both charge density and many-electron population numbers of
the -shell) of the equilibrium volume and the bulk modulus of selected
lanthanides have been performed and a good agreement is obtained.Comment: 1 fi
Charge and Orbital Ordering and Spin State Transition Driven by Structural Distortion in YBaCo_2O_5
We have investigated electronic structures of antiferromagnetic YBaCo_2O_5
using the local spin-density approximation (LSDA) + U method. The charge and
orbital ordered insulating ground state is correctly obtained with the strong
on-site Coulomb interaction. Co^{2+} and Co^{3+} ions are found to be in the
high spin (HS) and intermediate spin (IS) state, respectively. It is considered
that the tetragonal to orthorhombic structural transition is responsible for
the ordering phenomena and the spin states of Co ions. The large contribution
of the orbital moment to the total magnetic moment indicates that the
spin-orbit coupling is also important in YBaCo_2O_5.Comment: 4 pages including 4 figures, Submitted to Phys. Rev. Let
Electronic structure of the MO oxides (M=Mg, Ca, Ti, V) in the GW approximation
The quasiparticle band structures of nonmagnetic monoxides, MO (M=Mg, Ca, Ti,
and V), are calculated by the GW approximation. The band gap and the width of
occupied oxygen 2p states in insulating MgO and CaO agree with experimental
observation. In metallic TiO and VO, conduction bands originated from metal 3d
states become narrower. Then the partial densities of transition metal e_g and
t_2g states show an enhanced dip between the two. The effects of static
screening and dynamical correlation are discussed in detail in comparison with
the results of the Hartree-Fock approximation and the static Coulomb hole plus
screened exchange approximation. The d-d Coulomb interaction is shown to be
very much reduced by on-site and off-site d-electron screening in TiO and VO.
The dielectric function and the energy loss spectrum are also presented and
discussed in detail.Comment: 10 pages, 5 figure
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