V. P. Antropov

Electrons, phonons, and their interaction in YBa2Cu3O7

Abstract We have performed LDA calculations of structure, phonon frequencies, and the electron-phonon interaction. We find good agreement with available data and estimate: λ=1. Saddlepoints 20 meV below the Fermi level are important.

Electronic and magnetic structure of GdCu and GdZn

The electronic structure of the intermetallic compounds GdCu and GdZn has been calculated using density functional theory and the LMTO method for the ferromagnetic and two different antiferromagnetic orderings of the Gd local moments. The ground state of GdZn is found to be ferromagnetic while GdCu is antiferromagnetic, in agreement with experiment. The reason for the stability of one or other phase is discussed in terms of density of states features near the Fermi level. The effective exchange parameter J0 is determined from the Green functions corresponding to the calculated magnetic ground states. It is explained why the d-contribution to the Gd magnetic moment is enhanced in ferromagnetic GdZn.

Electron-Phonon Coupling, Coulomb Pseudo Potential µ* and Physical Properties of C60 Compounds

We present an ab initio calculation of the phonon frequencies and electron phonon-coupling for the intra molecular phonons of A g and H g symmetry. We obtain a coupling λ ~ 0.6 – 0.7 for K3C60 and Rb3C60. We estimate the Coulomb pseudopotential µ *. Within the random phase approximation the screened, but unrenormalized, µ ~ 0.4. To estimate the renormalization due to retardation effects, the inter band scattering matrix elements are calculated and found to be important. Within the framework where ladder diagrams in the screened interaction are summed, we therefore find that renormalization effects are important. Vertex corrections are, however, important, and for a two-band model, we show that, as a result, renormalization effects actually are small. Assuming that this also applies to C60 we obtain µ * ~ 0.4. To explain the superconductivity we then have to assume that the electron-phonon coupling is somewhat (factor 1.3) larger than the calculated value. The photoemission spectrum for doped C60 compounds is studied, including phonon and plasmon satellites. We show that the anomalously broad spectrum for K3C60 and Rb3C60 can be explained by the presence of plasmon satellites. Finally the phonon contribution to the T dependent resistivity is studied. A satisfactory description of the experimental results is obtained and some limits on the electron-phonon coupling are established.

High-Resolution Photoemission Studies of A3C60 (A = K, Rb)

We report high resolution photoemission spectra of A3C60 (A=K, Rb) recorded at low temperature. The spectra clearly show near Fermi-edge fine structure, as well as an anomalous width of the conduction band spectral weight and density of states at EF. Calculations of the photoemission spectra show that these features can be understood in terms of the presence of satellites due to both phonons and coupling to the charge carrier plasmon.

Orientational Order in A3C60: Effects on Valence Bands and Infrared Optical Spectra

By analysis of the electronic contribution to the binding in A3C60, we show that the problem of orientational ordering can be mapped onto the fcc nearest-neighbor antiferromagnetic Ising model with J of the order of 100K. The ground state crystal structure should be therefore anti-ferromagnetically ordered in two dimensions and disordered in the third. This is not inconsistent with present X-ray powder data. We find that the low-temperature conduction-band structure closely resembles that of the simplest antiferromagnetically ordered (so-called bi-directional) crystal. Calculated optical conductivity can be fitted by a Drude-like contribution and an “interband” Lorenzian peak at 400-500 cm−1.