Superlight bipolarons and a checkerboard d-wave condensate in cuprates

TL;DR

This paper proposes that a bipolaron-based extension of BCS theory explains high-temperature superconductivity in cuprates through superlight bipolarons forming a d-wave condensate with checkerboard modulations.

Contribution

It introduces a bipolaron model in the strong-coupling regime as a novel explanation for superconductivity in cuprates, emphasizing superlight bipolarons and their d-wave Bose-Einstein condensate.

Findings

Identification of superlight bipolarons as key carriers

Explanation of checkerboard modulation of hole density

Description of d-wave Bose-Einstein condensate in cuprates

Abstract

The seminal work by Bardeen, Cooper and Schrieffer taken further by Eliashberg to the intermediate coupling solved the problem of conventional superconductors about half a century ago. The Froehlich and Jahn-Teller electron-phonon interactions were identified as an essential piece of physics in all novel superconductors. The BCS theory provides a qualitatively correct description of some of them like magnesium diborade and doped fullerenes (if the polaron formation is taken into account). However, cuprates remain a problem. Here I show that the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution. Low-energy physics in this regime is that of small 'superlight' bipolarons, which are real-space mobile bosonic pairs dressed by phonons. The symmetry and space modulations of the order parameter are explained in the framework of the bipolaron theory. A d-wave Bose-Einstein condensate of bipolarons reveals itself as a checkerboard modulation of the hole density and of the gap below Tc.