Superlight small bipolarons: a route to room temperature superconductivity

TL;DR

This paper explores how superlight small bipolarons, formed via unscreened Froehlich EPI and Coulomb interactions, could enable room-temperature superconductivity, especially in layered ionic materials like cuprates.

Contribution

It predicts conditions for room-temperature superconductivity based on superlight bipolarons in a Coulomb-Froehlich model, extending previous theories with analytical and numerical insights.

Findings

Superlight bipolarons are several orders of magnitude lighter than in traditional models.

Certain material conditions favor the formation of superlight bipolarons suitable for high Tc.

Most conditions for superlight bipolaron formation are already present in cuprates.

Abstract

Extending the BCS theory towards the strong electron-phonon interaction (EPI), a charged Bose liquid of small bipolarons has been predicted by us with a further prediction that the highest superconducting critical temperature is found in the crossover region of the EPI strength from the BCS-like to bipolaronic superconductivity. Later on we have shown that the unscreened (infinite-range) Froehlich EPI combined with the strong Coulomb repulsion create \emph{superlight} small bipolarons, which are several orders of magnitude lighter than small bipolarons in the Holstein-Hubbard model (HHM) with a zero-range EPI. The analytical and numerical studies of this Coulomb-Froehlich model (CFM) provide the following recipes for room-temperature superconductivity: (a) The parent compound should be an ionic insulator with light ions to form high-frequency optical phonons, (b) The structure should be quasi two-dimensional to ensure poor screening of high-frequency phonons polarized perpendicular to the conducting planes, (c) A triangular lattice is required in combination with strong, on-site Coulomb repulsion to form the small superlight bipolaron, (d) Moderate carrier densities are required to keep the system of small bipolarons close to the Bose-Einstein condensation regime. Clearly most of these conditions are already met in the cuprates.