Phase separation of electrons strongly coupled with phonons in cuprates and manganites

TL;DR

This paper proposes that strong electron-phonon interactions combined with disorder cause phase separation and pairing phenomena in cuprates and manganites, challenging the notion that Hubbard models alone explain these effects.

Contribution

It introduces a mechanism where electron-phonon interactions lead to phase separation and pairing in cuprates and manganites, emphasizing the role of disorder and bipolarons.

Findings

Electron-phonon interactions induce phase separation in cuprates and manganites.

Heavy bipolarons form and explain CMR and phase coexistence.

Disorder influences localization and propagation of charge carriers.

Abstract

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron-electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron-phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott-Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localised by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Froehlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.