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Superlight bipolarons and criterion of BCS-BEC crossover in cuprates

A. S. Alexandrov
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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.

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A. S. Alexandrov. 2003-01-28. Superlight bipolarons and criterion of BCS-BEC crossover in cuprates. https://doi.org/10.1142/s0217979203020910

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