LaFeAsO1−x_{1-x}Fx_x: A low carrier density superconductor near itinerant magnetism

Density functional studies of 26K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, $N(E_F)$ and modest phonon frequencies relative to $T_c$. The high $N(E_F)$ leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by doping level. Thus LaFeAs(O,F) is in a unique class of high $T_c$ superconductors: high $N(E_F)$ ionic metals near magnetism.Comment: Shortened published form. Typos correcte

A Tight-Binding Investigation of the NaxCoO2 Fermi Surface

We perform an orthogonal basis tight binding fit to an LAPW calculation of paramagnetic Na$_x$CoO$_2$ for several dopings. The optimal position of the apical oxygen at each doping is resolved, revealing a non-trivial dependence of the band structure and Fermi surface on oxygen height. We find that the small e$_{g'}$ hole pockets are preserved throughout all investigated dopings and discuss some possible reasons for the lack of experimental evidence for these Fermi sheets

Electronic Structure and Thermoelectric Prospects of Phosphide Skutterudites

The prospects for high thermoelectric performance in phosphide skutterudites are investigated based on first principles calculations. We find that stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in that it is metallic. As such the band structure must be modified if high thermopowers are to be achieved. In analogy to the antimonides it is expected that this may be done by filling with La. Calculations for LaFe_4P_12 show that a gap can in fact be opened by La filling, but that the valence band is too light to yield reasonable p-type thermopowers at appropriate carrier densities; n-type La filled material may be more favorable.Comment: 3 pages, 3 figures, 1 tabl

Density functional study of FeS, FeSe and FeTe: Electronic structure, magnetism, phonons and superconductivity

We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum, magnetism and electron-phonon coupling for the superconducting phase FeSe, as well as the related compounds FeS and FeTe. We find that the Fermi surface structure of these compounds is very similar to that of the Fe-As based superconductors, with cylindrical electron sections at the zone corner, cylindrical hole surface sections, and depending on the compound, other small hole sections at the zone center. As in the Fe-As based materials, these surfaces are separated by a 2D nesting vector at ($\pi$,$\pi$). The density of states, nesting and Fermi surface size increase going from FeSe to FeTe. Both FeSe and FeTe show spin density wave ground states, while FeS is close to an instability. In a scenario where superconductivity is mediated by spin fluctuations at the SDW nesting vector, the strongest superconductor in this series would be doped FeTe.Comment: Added note regarding recent experimental observations of superconductivity under pressure. Some additional discussio

Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing

Ru based perovskites demonstrate an amazing richness in their magnetic properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and unconventional superconductivity. Tendency to ferromagnetism, stemming from the unusually large involvement of O in magnetism in ruthenates, leads to ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave superconductivity. A related compound Ca2RuO4 was measured to be antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations in Sr2RuO4 as well. Here we report first principles calculations that demonstrate that in both compounds the ferro- and antiferromagnetic fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a close competition between p-wave and d-wave superconducting symmetries in Sr2RuO4. The antiferromagnetism in this system appears to be mostly related with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly, for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one everywhere except in close vicinity of the antiferromagnetic instability. The most stable state within the d-wave channel has vanishing order parameter at one out of three Fermi surfaces in Sr2RuO4, while in the p channel its amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are color, but should look OK in B&W as wel

Cohesion of BaReH9_9 and BaMnH9_9: Density Functional Calculations and Prediction of (MnH9)2−_9)^{2-} Salts

Density functional calculations are used to calculate the structural and electronic properties of BaReH$_9$ and to analyze the bonding in this compound. The high coordination in BaReH$_9$ is due to bonding between Re 5$d$ states and states of $d$-like symmetry formed from combinations of H $s$ orbitals in the H$_9$ cage. This explains the structure of the material, its short bond lengths and other physical properties, such as the high band gap. We compare with results for hypothetical BaMnH$_9$, which we find to have similar bonding and cohesion to the Re compound. This suggests that it may be possible to synthesize (MnH$_9)^{2-}$ salts. Depending on the particular cation, such salts may have exceptionally high hydrogen contents, in excess of 10 weight

Investigation of the effects of cobalt ions on epoxy properties

The effects of Co(acac)sub x complexes on MY-720 epoxy properties have been investigated. It appears that Co2(+) ions form antibonding or nonbonding orbitals which increase the free volume and also reduce the cohesiveness of the host epoxy. The effects of Co2(+) ions, on the other hand, seem to result in increased Cohesiveness of the epoxy. The experimental values of magnetic moments of both types of ions in MY-720 suggest that the orbital momentum contributions of the (3d) electrons are partially conserved, though the effect is more pronounced for Co2(+) ions. The coordination environment of the cobalt ions in the host epoxy does not appear to be uniquely defined. These results indicate that the effects of metal ions on resin properties cannot be easily predicted on the basis of ligand field theory argument alone. Complex interactions between metal ions and host epoxy molecular structure suggest the desirability of parallel experimental investigations of electronic, magnetic, and mechanical properties of metal ion-containing epoxy samples for comparison with theory

First Principles Study of Zn-Sb Thermoelectrics

We report first principles LDA calculations of the electronic structure and thermoelectric properties of $\beta$-Zn$_{4}$Sb$_{3}$. The material is found to be a low carrier density metal with a complex Fermi surface topology and non-trivial dependence of Hall concentration on band filling. The band structure is rather covalent, consistent with experimental observations of good carrier mobility. Calculations of the variation with band filling are used to extract the doping level (band filling) from the experimental Hall number. At this band filling, which actually corresponds to 0.1 electrons per 22 atom unit cell, the calculated thermopower and its temperature dependence are in good agreement with experiment. The high Seebeck coefficient in a metallic material is remarkable, and arises in part from the strong energy dependence of the Fermiology near the experimental band filling. Improved thermoelectric performance is predicted for lower doping levels which corresponds to higher Zn concentrations.Comment: 5 pages, 6 figure