Jahn–Teller Polarons, Bipolarons and Inhomogeneities. A Possible Scenario for Superconductivity in Cuprates

Abstract

Some of the early models of high temperature superconductivity (HTS) in cuprates dismissed a pairing mechanism based on electron–phonon (e–ph) interactions. One of the arguments against the e–ph theories was the negligible isotope effect on the critical temperature, T c. Other arguments were based on approximations performed near the strong e–ph interaction regime in which HTS might take place1. This leads to the conclusion that an e–ph paring is inoperative. As a result, pure electron correlations, excitonic mechanisms and spin fluctuations have attracted most of the attention, overshadowing the e–ph approaches. However, some of the features shown by copper oxides seem to validate the e–ph models, in particular those which concern small bipolarons and the Jahn–Teller (JT) effect. For instance, these materials have a bandwidth within a range where the strength of JT coupling is important. Nonadiabatic effects cannot be ignored when high frequency phonons are coupled to itinerant charges. Therefore, theories based on on-site or intersite bipolarons, JT bipolarons and different mechanisms of carrier dynamics, such as Bose-Einstein condensation and tunneling-percolation, have been proposed. However, our discussion is centered on the JT models and the intriguing possibility that HTS could be driven by JT forces. The JT models have several distinctive features: they deal with a multidimensional electron basis coupled to symmetric phonon degrees of freedom. Moreover, JT Polarons exhibit strong anharmonicity. The review highlights these local constraints and their consequences for the dynamics of polaron formation, the appearance of an inhomogeneous state and charge transport properties. In addition, the broken local symmetry at the thermodynamic limit of interacting JT polarons, when it is combined with long range Coulomb interactions, leads to specific macroscopic manifestations. Thus, cooperative effects go beyond standard structural transitions, and a novel organization of nanoscopic textures is manifested. Here we also address this issue and its connection with HTS, e.g. whether the so-called pseudogap observed in cuprates is related to the energy scale of the JT-bipolaron formation and the phase-segregation phase.