Upper critical field of bipolaron superconductors

In narrow-band crystals with strong electron-phonon interactions, electrons form bound pairs that are localized at lattice sites, i.e., small-radius bipolarons. The authors show that for arbitrary concentrations of these pairs the properties of these systems can be described using an equivalent Bose Hamiltonian. For arbitrary bipolaron concentrations they find the upper critical magnetic field Hc2 (T), including anisotropy. Their predictions of the temperature dependence of the field Hc2 explain the unusual behavior of this field in the high-temperature superconductors