Photoemission and magnetic response in the bipolaronic superconductor

A theory of angle-resolved photoemission (ARPES) in doped charge-transfer Mott insulators is developed taking into account the realistic band structure, (bi)polaron formation due to the strong electron-phonon interaction, and a random field potential. We derive the coherent part of the ARPES spectra with the oxygen hole spectral function calculated in the non-crossing (ladder) approximation and with the exact spectral function of a one-dimensional hole in a random potential. On the basis of this theory, explanations are proposed for several features of the ARPES spectra taken from the cuprate superconductors. These include the polarisation dependence and spectral shape in YBa2Cu3O7 and YBa2Cu4O8. The theory is compatible with the doping dependence of kinetic and thermodynamic properties of the cuprates as well as with the d-wave symmetry of the superconducting order parameter. The scattering cross section of a Coulomb potential screened by a charged Bose gas (CBG) is calculated both above and below the Bose-Einstein condensation temperature, using the variable phase method. In contrast with the BCS superconductor, the screened scattering potential and quasiparticle lifetime are found to be very different in the superconducting and normal states. We apply the result to explain the appearance of a sharp peak in the ARPES spectra in some cuprates below the superconducting transition. The charged Bose gas model for the upper critical field in the cuprates is extended to explain the crossing point in the curves of induced magnetization divided by the square root of field against temperature in the less anisotropic cuprates. This model has already been shown to provide a parameter-free expression for Tc in a wide range of cuprates. We compare our results with experiment in YBa2Cu3O7-δ. (author)