Intercalant-Driven Superconductivity in YbC6_{6} and CaC6_{6}

Recently deiscovered superconductivity in YbC$_6$ and CaC$_6$ at temperatures substantially higher than previously known for intercalated graphites, raised several new questions: (1) Is the mechanism considerably different from the previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given extreme similarity between YbC$_6$ and CaCa$_6$, why their critical temperatures are so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC$_6$ and CaC$_6$, but not in alkaline-intercalated compounds, and explain the difference in $T_c$ by the ``isotope effect'' due to the difference in Yb and Ca atomic masses.Comment: 4 pages, embedded postscript figire

Transport, optical and electronic properties of the half metal CrO2

The electronic structure of CrO_2 is critically discussed in terms of the relation of existing experimental data and well converged LSDA and GGA calculations of the electronic structure and transport properties of this half metal magnet, with a particular emphasis on optical properties. We find only moderate manifestations of many body effects. Renormalization of the density of states is not large and is in the typical for transition metals range. We find substantial deviations from Drude behavior in the far-infrared optical conductivity. These appear because of the unusually low energy of interband optical transitions. The calculated mass renormalization is found to be rather sensitive to the exchange-correlation functional used and varies from 10% (LSDA) to 90% (GGA), using the latest specific heat data. We also find that dressing of the electrons by spin fluctuations, because of their high energy, renormalizes the interband optical transition at as high as 4 eV by about 20%. Although we find no clear indications of strong correlations of the Hubbard type, strong electron-magnon scattering related to the half metallic band structure is present and this leads to a nontrivial temperature dependence of the resistivity and some renormalization of the electron spectra.Comment: 9 Revtex 2 column pages, including 8 postscript figures. Two more figures are included in the submission that are not embedded in the paper, representing DOS and bandstructure of the paramagnetic CrO

Effects of Crystal Structure and the On-Site Coulomb Interactions on the Electronic and Magnetic Structure of Pyrochlores A2A_2Mo2_2O7_7 (A= Y, Gd, and Nd)

Being motivated by recent experimental studies, we investigate magnetic structures of the Mo pyrochlores $A_2$Mo$_2$O$_7$ ($A$= Y, Nd, and Gd) and their impact on the electronic properties. The latter are closely related with the behavior of twelve Mo($t_{2g}$) bands, located near the Fermi level and well separated from the rest of the spectrum. We use a mean-field Hartree-Fock approach, which combines fine details of the electronic structure for these bands, extracted from the conventional calculations in the local-density approximation, the spin-orbit interaction, and the on-site Coulomb interactions amongst the Mo(4d) electrons, treated in the most general rotationally invariant form. The Coulomb repulsion U plays a very important role in the problem, and the semi-empirical value U$\sim$1.5-2.5 eV accounts simultaneously for the metal-insulator (M-I) transition, the ferromagnetic (FM) - spin-glass (SG) transition, and for the observed enhancement of the anomalous Hall effect (AHE). The M-I transition is mainly controlled by $U$. The magnetic structure at the metallic side is nearly collinear FM, due to the double exchange mechanism. The transition into the insulating state is accompanied by the large canting of spin and orbital magnetic moments. The sign of exchange interactions in the insulating state is controlled by the Mo-Mo distances. Smaller distances favor the antiferromagnetic coupling, which preludes the SG behavior in the frustrated pyrochlore lattice. Large AHE is expected in the nearly collinear FM state, near the point of M-I transition, and is related with the unquenched orbital magnetization at the Mo sites. We also predict large magneto-optical effect in the same FM compounds.Comment: 26 pages, 17 figures (low resolution is used for Figs. 6, 8, and 9, please contact directly if you need the originals), 1 tabl