Search

Using previously reported first-principle frozen-phonon LDA calculations we have calculated the effective superconducting and transport electron-phonon coupling constants. We have taken into account the anisotropy due to the difference between four bands crossing the Fermi level in YBa2Cu3O7, and found that because the anisotropy influence both electron-phonon coupling constants, but in the opposite direction, the superconducting constant becomes larger than the transport one. The absolute values of the effective coupling constants is close to 2. The superconducting gaps for the different bands vary by nearly an order of magnitude. We also discuss how various experiments might have been explained in terms of the interband anisotropy.

We have calculated the renormalization of the specific heat in the normal state and the specific heat jump at Tc, using a realistic Eliashberg function, extracted from tunneling spectroscopy. The magnitude of the Eliashberg function was scaled so as to have the total coupling constant λ from 1.5 to 2.5. Using these calculations, we have analyzed recent measurements of the specific heat and have found that they can be reconciled with the band structure calculations, if λ≈2.

We have studied theoretically a two-band superconductor in which superconductivity is induced by a strong retarded pairing interaction in one of the bands and has s-wave symmetry. This picture results in strongly anisotropic order parameter. Some consequencies of the model are discussed.

Measurements of the absolute electronic Raman cross sections are presented for a single domain Y-123 crystal for different polarization geometries. They are compared to the corresponding inverse effective mass fluctuations evaluated from the LMTO full-potential band structure of O.K. Andersen et al . The agreement of the relative intensities for the xx - yy anisotropy and between the crossed polarizations parallel and at 45° to the crystal ab axes, i.e. B 2 g and B 1 g symmetries, with the LDA single particle masses is good and suggests that isotropic and anisotropic density fluctuations involving various sheets and regions of the Fermi surface are responsible for the electronic Raman continuum. Resonance effects and screening are taken into account.

We report the LDA+ U f calculations of the electronic structure of ABa 2 Cu 3 O 7 , (A=Pr, Nd). For A=Pr an additional band crosses the Fermi level, grabbing holes from the (super) conducting CuO 2 band. For A=Nd this band remains completely filled, thus explaining the absence of T c suppression. On doping Ln Ba 2 Cu 3 O 7 with Pr the position of the hole-depleting band depends crucially of the atomic number of the host rare earth, through the position of its f -band. This makes T c supression in doped compounds different for different hosts, the stronger the smaller the atomic number of the host is.

The Hopfield factor (electronic part of the electron-phonon coupling constant) is calculated in the Rigid-Muffin-Tin approximation (RMTA) for several perovskite high-T c superconductors. Calculations were performed in the conventional “spherical” approximation, as well as taking into account proper local symmetry. It is shown that the layered perovskite crystal symmetry generally favors strong electron-phonon interaction on copper site. We also estimate some for non-RMTA effects.