Strong-coupling theory of high-temperature superconductivity and colossal magnetoresistance

TL;DR

This paper extends the BCS theory to strong-coupling regimes to explain high-temperature superconductivity in cuprates and colossal magnetoresistance in ferromagnetic oxides, emphasizing phonon interactions and bipolarons.

Contribution

It introduces a strong-coupling theory that unifies the understanding of high-Tc superconductivity and CMR phenomena through phonon dressing and bipolaron formation.

Findings

Long-range Froehlich electron-phonon interaction is crucial in cuprates.

Heavy bipolarons explain pairing and phase transitions in manganites.

The theory accounts for pseudogaps, isotope effects, and resistivity behavior.

Abstract

We argue that the extension of the BCS theory to the strong-coupling regime describes the high-temperature superconductivity of cuprates and the colossal magnetoresistance (CMR) of ferromagnetic oxides if the phonon dressing of carriers and strong attractive correlations are taken into account. The long-range Froehlich electron-phonon interaction has been identified as the most essential in cuprates providing "superlight" lattice polarons and bipolarons. Here some kinetic, magnetic, and more recent thermomagnetic normal state measurements are interpreted in the framework of the strong-coupling theory, including the Nernst effect and normal state diamagnetism. Remarkably, a similar strong-coupling approach offers a simple explanation of CMR in ferromagnetic oxides. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase account for the first-order ferromagnetic phase transition, CMR, isotope effects, and pseudogaps in doped manganites. Here we propose an explanation of the phase coexistence and describe the shape of resistivity of manganites near the transition in the framework of the strong-coupling approach.