Lattice polarization effects on the screened Coulomb interaction WW 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 $GW$ self-energy where $W$ 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 $1/q^2$ so that the predominant contribution comes from a small region around $q{=}0$. 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 $q$ near $q{=}0$. We estimate the magnitude for $\mathbf{q}{\rightarrow}0$ 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 $-\alpha_P\omega_L/2$ 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 CaFe2_{2}As2_{2}

Using linear-response density-functional theory, magnetic excitations in the striped phase of CaFe$_{2}$As$_{2}$ 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 $d$ 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

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

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 Ni$_{80}$Fe$_{20}$ 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

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

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

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