971 research outputs found
Ab Initio Calculation of Crystalline Electric Fields and Kondo Temperatures in Ce-Compounds
We have calculated the band- hybridizations for CeLaM
compounds ( and ; M=Pb, In, Sn, Pd) within the local
density approximation and fed this into a non-crossing approximation for the
Anderson impurity model applied to both dilute and concentrated limits. Our
calculations produce crystalline electric field splittings and Kondo
temperatures with trends in good agreement with experiment and demonstrate the
need for detailed electronic structure information on hybridization to describe
the diverse behaviors of these Ce compounds.Comment: 13 pages(RevTeX), 3 Postscript figure
Low temperature spin diffusion in the one-dimensional quantum nonlinear -model
An effective, low temperature, classical model for spin transport in the
one-dimensional, gapped, quantum non-linear -model is developed.
Its correlators are obtained by a mapping to a model solved earlier by Jepsen.
We obtain universal functions for the ballistic-to-diffusive crossover and the
value of the spin diffusion constant, and these are claimed to be exact at low
temperatures. Implications for experiments on one-dimensional insulators with a
spin gap are noted.Comment: 4 pages including 3 eps-figures, Revte
Velocity Correlations, Diffusion and Stochasticity in a One-Dimensional System
We consider the motion of a test particle in a one-dimensional system of
equal-mass point particles. The test particle plays the role of a microscopic
"piston" that separates two hard-point gases with different concentrations and
arbitrary initial velocity distributions. In the homogeneous case when the
gases on either side of the piston are in the same macroscopic state, we
compute and analyze the stationary velocity autocorrelation function C(t).
Explicit expressions are obtained for certain typical velocity distributions,
serving to elucidate in particular the asymptotic behavior of C(t). It is shown
that the occurrence of a non-vanishing probability mass at zero velocity is
necessary for the occurrence of a long-time tail in C(t). The conditions under
which this is a tail are determined. Turning to the inhomogeneous
system with different macroscopic states on either side of the piston, we
determine its effective diffusion coefficient from the asymptotic behavior of
the variance of its position, as well as the leading behavior of the other
moments about the mean. Finally, we present an interpretation of the effective
noise arising from the dynamics of the two gases, and thence that of the
stochastic process to which the position of any particle in the system reduces
in the thermodynamic limit.Comment: 22 files, 2 eps figures. Submitted to PR
Spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures
We present the theory of nonzero temperature () spin dynamics and
transport in one-dimensional Heisenberg antiferromagnets with an energy gap
. For , we develop a semiclassical picture of thermally
excited particles. Multiple inelastic collisions between the particles are
crucial, and are described by a two-particle S-matrix which has a
super-universal form at low momenta. This is established by computations on the
O(3) -model, and strong and weak coupling expansions (the latter using
a Majorana fermion representation) for the two-leg S=1/2 Heisenberg
antiferromagnetic ladder. As an aside, we note that the strong-coupling
calculation reveals a S=1, two particle bound state which leads to the presence
of a second peak in the T=0 inelastic neutron scattering (INS) cross-section
for a range of values of momentum transfer. We obtain exact, or numerically
exact, universal expressions for the thermal broadening of the quasi-particle
peak in the INS cross-section, for the magnetization transport, and for the
field dependence of the NMR relaxation rate of the effective
semiclassical model: these are expected to be asymptotically exact for the
quantum antiferromagnets. The results for are compared with the
experimental findings of Takigawa et al and the agreement is quite good. In the
regime we argue that a
complementary description in terms of semiclassical waves applies, and give
some exact results for the thermodynamics and dynamics.Comment: REVTEX, 53 pages and 23 postscript figures; added additional
reference and associated clarificatio
Composition Dependence of the Structure and Electronic Properties of Liquid Ga-Se Alloys Studied by Ab Initio Molecular Dynamics Simulation
Ab initio molecular dynamics simulation is used to study the structure and
electronic properties of the liquid Ga-Se system at the three compositions
GaSe, GaSe and GaSe, and of the GaSe and GaSe crystals. The
calculated equilibrium structure of GaSe crystal agrees well with available
experimental data. The neutron-weighted liquid structure factors calculated
from the simulations are in reasonable agreement with recent neutron
diffraction measurements. Simulation results for the partial radial
distribution functions show that the liquid structure is closely related to
that of the crystals. A close similarity between solid and liquid is also found
for the electronic density of states and charge density. The calculated
electronic conductivity decreases strongly with increasing Se content, in
accord with experimental measurements.Comment: REVTeX, 8 pages and 12 uuencoded PostScript figures, submitted to
Phys. Rev. B. corresponding author: [email protected]
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
A bit-rate flexible and power efficient all-optical demultiplexer realised by monolithically integrated Michelson interferometer
Rare earth contributions to the X-ray magnetic circular dichroism at the Co K edge in rare earth-cobalt compounds investigated by multiple-scattering calculations
The X-ray magnetic circular dichroism (XMCD) has been measured at the Co K
edge in Co-hcp and R-Co compounds (R=La, Tb, Dy). The structure of the
experimental XMCD spectra in the near-edge region has been observed to be
highly sensitive to the magnetic environment of the absorbing site.
Calculations of the XMCD have been carried out at the Co K edge in Co metal,
LaCo and TbCo within the multiple-scattering framework including the
spin-orbit coupling. In the three systems, the XMCD spectra in the near-edge
region are well reproduced. The possibility to separate and quantitatively
estimate the local effects from those due to the neighboring atoms in the XMCD
cross section makes possible a more physical understanding of the spectra. The
present results emphasize the major role played by the states of the Tb
ions in the XMCD spectrum at the Co K edge in the TbCo compound.Comment: 34 pages, revtex, 10 eps figures included with epsf, after referee
revie
Magnetocrystalline Anisotropy Energy of Transition Metal Thin Films: A Non-perturbative Theory
The magnetocrystalline anisotropy energy E(anis) of free-standing monolayers
and thin films of Fe and Ni is determined using two different semi-empirical
schemes. Within a tight-binding calculation for the 3d bands alone, we analyze
in detail the relation between bandstructure and E(anis), treating spin-orbit
coupling non-pertubatively. We find important contributions to E(anis) due to
the lifting of band degeneracies near the Fermi level by SOC. The important
role of degeneracies is supported by the calculation of the electron
temperature dependence of the magnetocrystalline anisotropy energy, which
decreases with the temperature increasing on a scale of several hundred K. In
general, E(anis) scales with the square of the SOC constant. Including 4s bands
and s-d hybridization, the combined interpolation scheme yields anisotropy
energies that quantitatively agree well with experiments for Fe and Ni
monolayers on Cu(001). Finally, the anisotropy energy is calculated for systems
of up to 14 layers. Even after including s-bands and for multilayers, the
importance of degeneracies persists. Considering a fixed fct-Fe structure, we
find a reorientation of the magnetization from perpendicular to in-plane at
about 4 layers. For Ni, we find the correct in-plane easy-axis for the
monolayer. However, since the anisotropy energy remains nearly constant, we do
not find the experimentally observed reorientation.Comment: 15 pages, Revtex, 15 postscript figure
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