Isotropic Bipolaron–Fermion Exchange Theory and Unconventional Pairing in Cuprate Superconductors

Abstract

The appearance of unconventional pairing in superconducting cuprates is examined from a microscopic model, taking into account important properties of hole-doped copper oxides. An exchange interaction between fermions and dominantly inter-site bipolarons is considered to be the mechanism which leads to the pairing. Its momentum dependency is connected to the well-established fermion-phonon anomalies in cuprate superconductors. Since charge carriers in these materials are strongly correlated, a screened Coulomb repulsion is added to this exchange term. Any ad hoc assumptions like anisotropy are avoided, but a microscopic explanation of unconventional pairing for coupling strengths that are in accordance with experimental facts is provided. One important outcome is a mathematically rigorous elucidation of the role of Coulomb repulsion in unconventional pairing, which is shown to be concomitant with a strong depletion of superconducting pairs. The theory, applied to the special case of LaSr 214, predicts at optimal doping i) a coherence length of 21Å, which is the same as that obtained from the Ginzburg-Landau critical magnetic field measured for this material, and ii) d-wave pair formation in the pseudogap regime, that is, at temperatures much higher than the superconducting transition temperature.

Existência de pares \"d-wave\" e ondas de densidade em uma classe de modelos microscópicos para supercondutores com alta temperatura de transição

“…Pode estudar-se também métodos perturbativos para tratar o caso de bipolarons não localizados, como foi feito com auxilio de cálculo numérico para o problema de três corpos [39]. Para o problema coletivo não há, neste caso, invariância por permutação de blocos para o regime de acoplamento forte.…”