Possible explanation of the high-temperature superconductivity in complex oxides

A novel version of the polaronic mechanism of superconductivity is proposed based on the assumption of weak-coupling polarons existence with a considerable dipole moment in complex oxide systems. As an approach to the electrons, giving rise to the metallic conductivity of the Y-Ba-Cu-O materials, the free electron model is used. The free electrons with opposite wave vectors and momenta form weak-coupling polarons with opposite dipole moments. The electrons around the Fermi level, having turned into polarons through interaction with the optic phonons, pair into bipolarons due to dipole-dipole interraction of weak-coupling polarons, or will form a condensate of a large number of polarons. According to the model proposed the polaron condensate or bipolaron gas play a role of the Bose-Einstein condensate in conventional superconductors and will cause the superconductivity of the Y-Ba-Cu-O group ceramic materials. The critical temperature for transition into superconducting state could be approximately derived from kTc≅ Δ0. The transition into a superconducting state in high-temperature superconducting ceramic materials could be expected to arise only above a certain critical free electron concentration nc which is 1020 - 1021 cm-3. At Tc about 100 K and reasonable assumptions for r0, m* and Pp acceptable values for polaron total energy and its kinetic energy are obtained. The temperature dependence of the energy gap Δ(T) could be formulated from the proposed simple phenomenological model