356 research outputs found
Lattice polarization effects on the screened Coulomb interaction of the GW approximation
In polar insulators where longitudinal and transverse optical phonon modes
differ substantially, the electron-phonon coupling affects the energy-band
structure primarily through the long-range Fr\"ohlich contribution to the Fan
term. This diagram has the same structure as the self-energy where
originates from the electron part of the screened coulomb interaction. The two
can be conveniently combined by combining electron and lattice contributions to
the polarizability. Both contributions are nonanalytic at the origin, and
diverge as so that the predominant contribution comes from a small
region around . Here we adopt a simple estimate for the Fr\"ohlich
contribution by assuming that the entire phonon part can be attributed to a
small volume of near . We estimate the magnitude for
from a generalized Lyddane-Sachs-Teller relation,
and the radius from the inverse of the polaron length scale. The gap correction
is shown to agree with Fr\"ohlich's simple estimate of
the polaron effect
First-principles analysis of spin-disorder resistivity of Fe and Ni
Spin-disorder resistivity of Fe and Ni and its temperature dependence are
analyzed using noncollinear density functional calculations within the
supercell method. Different models of thermal spin disorder are considered,
including the mean-field approximation and the nearest-neighbor Heisenberg
model. Spin-disorder resistivity is found to depend weakly on magnetic
short-range order. If the local moments are kept frozen at their
zero-temperature values, very good agreement with experiment is obtained for
Fe, but for Ni the resistivity at elevated temperatures is significantly
overestimated. Agreement with experiment for Fe is improved if the local
moments are iterated to self-consistency. The overestimation of the resistivity
for paramagnetic Ni is attributed to the reduction of the local moments down to
0.35 Bohr magnetons. Overall, the results suggest that low-energy spin
fluctuations in Fe and Ni are better viewed as classical rotations of local
moments rather than quantized spin fluctuations that would require an (S+1)/S
correction.Comment: 10 pages (RevTeX), 6 eps figure
Low Energy, Coherent, Stoner-like Excitations in CaFeAs
Using linear-response density-functional theory, magnetic excitations in the
striped phase of CaFeAs are studied as a function of local moment
amplitude. We find a new kind of excitation: sharp resonances of Stoner-like
(itinerant) excitations at energies comparable to the N{\'{e}}el temperature,
originating largely from a narrow band of Fe states near the Fermi level,
and coexist with more conventional (localized) spin waves. Both kinds of
excitations can show multiple branches, highlighting the inadequacy of a
description based on a localized spin model
The effects of interface morphology on Schottky barrier heights: a case study on Al/GaAs(001)
The problem of Fermi-level pinning at semiconductor-metal contacts is
readdressed starting from first-principles calculations for Al/GaAs. We give
quantitative evidence that the Schottky barrier height is very little affected
by any structural distortions on the metal side---including elongations of the
metal-semiconductor bond (i.e. interface strain)---whereas it strongly depends
on the interface structure on the semiconductor side. A rationale for these
findings is given in terms of the interface dipole generated by the ionic
effective charges.Comment: 5 pages, latex file, 2 postscript figures automatically include
Direct Method for Calculating Temperature-Dependent Transport Properties
We show how temperature-induced disorder can be combined in a direct way with
first-principles scattering theory to study diffusive transport in real
materials. Excellent (good) agreement with experiment is found for the
resistivity of Cu, Pd, Pt (and Fe) when lattice (and spin) disorder are
calculated from first principles. For Fe, the agreement with experiment is
limited by how well the magnetization (of itinerant ferromagnets) can be
calculated as a function of temperature. By introducing a simple Debye-like
model of spin disorder parameterized to reproduce the experimental
magnetization, the temperature dependence of the average resistivity, the
anisotropic magnetoresistance and the spin polarization of a NiFe
alloy are calculated and found to be in good agreement with existing data.
Extension of the method to complex, inhomogeneous materials as well as to the
calculation of other finite-temperature physical properties within the
adiabatic approximation is straightforward.Comment: Accepted as a Rapid Communication in Physical Review
Many-body Electronic Structure of Metallic alpha-Uranium
We present results for the electronic structure of alpha uranium using a
recently developed quasiparticle self-consistent GW method (QSGW). This is the
first time that the f-orbital electron-electron interactions in an actinide has
been treated by a first-principles method beyond the level of the generalized
gradient approximation (GGA) to the local density approximation (LDA). We show
that the QSGW approximation predicts an f-level shift upwards of about 0.5 eV
with respect to the other metallic s-d states and that there is a significant
f-band narrowing when compared to LDA band-structure results. Nonetheless,
because of the overall low f-electron occupation number in uranium,
ground-state properties and the occupied band structure around the Fermi energy
is not significantly affected. The correlations predominate in the unoccupied
part of the f states. This provides the first formal justification for the
success of LDA and GGA calculations in describing the ground-state properties
of this material.Comment: 4 pages, 3 fihgure
Physical and mathematical theories of tile and ditch drainage and their usefulness in design
A number of theories for tile and ditch drainage have been proposed in recent years which, if valid, would enable the rational design of many drainage systems. Nevertheless, most drainage systems are still designed by rule of thumb based largely upon the observations of technicians with experience in certain restricted areas.
To develop a theoretically sound and practically valuable method of designing subsurface drainage systems, the various approaches which have been made should be critically evaluated and compared, mutually, as well as with field data. However, no such analysis has been found in the literature.
The object of this publication is to provide this type of appraisal. The assumptions underlying a number of methods of analysis will be scrutinized in detail, and various applications of these methods to field results will be tested. It is hoped that this evaluation of the status quo will be useful in determining to what extent present theories lend themselves to field applications and what phases of drainage design need further study.
In general, this discussion will be restricted to problems of saturated flow, while recognizing that flow in the unsaturated zone above the water table often may be important. Little progress has been made in formulating quantitative theories regarding flow in the unsaturated zone
The Electronic Correlation Strength of Pu
An electronic quantity, the correlation strength, is defined as a necessary
step for understanding the properties and trends in strongly correlated
electronic materials. As a test case, this is applied to the different phases
of elemental Pu. Within the GW approximation we have surprisingly found a
"universal" scaling relationship, where the f-electron bandwidth reduction due
to correlation effects is shown to depend only on the local density
approximation bandwidth and is otherwise independent of crystal structure and
lattice constant.Comment: 7 pages, 4 figures, This version of the paper has been revised to add
additional background informatio
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