Superlight bipolarons and criterion of BCS-BEC crossover in cuprates

TL;DR

This paper develops a bipolaron model considering long-range interactions to explain high-temperature superconductivity in cuprates, challenging some existing criteria of the BCS-BEC crossover.

Contribution

It introduces a long-range interaction-based bipolaron model that accounts for key features of cuprates and clarifies the nature of pairing in these materials.

Findings

Real-space pairing occurs in many cuprates.

Long-range interactions are crucial for understanding HTSC.

Contradicts some criteria of the BCS-BEC crossover.

Abstract

Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Froehlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. Here it is shown that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.