d-wave Bose-Einstein condensate and tunnelling in superconducting cuprates

Starting from the Hamiltonian, which describes holes in a doped Mott insulator with the strong electron-phonon and Coulomb interactions we show that bipolaron formation leads to a d-wave charged Bose-Einstein condensate in cuprates. It is the bipolaron energy dispersion rather than a particular pairing interaction which is responsible for the d-wave symmetry. Single-particle spectral density is derived taking into account realistic band structure and disorder in cuprates. The tunnelling and photoemission (PES) spectra are described, including the temperature independent gap observed both in the superconducting and normal states, the emission/injection asymmetry, the finite zero-bias conductance, the spectral shape in the gap region and its temperature and doping dependence, the dip-hump incoherent asymmetric features at high voltage (tunnelling) and large binding energy (PES). The interaction responsible for the high value of T-c is elucidated which is the Frohlich electron-phonon interaction. (C) 1998 Elsevier Science B.V. All rights reserved.