6 - Formation, phase separation and superconductivity of large bipolarons

Summary

Abstract

This paper presents a scenario in which large (multi-site) bipolarons form and give rise to superconductivity. First the physical circumstances in which large bipolarons can form are elucidated. Then several identifying properties of large bipolarons are discussed. Finally, a model of how interactions between large bipolarons lead to their superconductivity is presented. I emphasize the existence of a phonon-mediated intermediate-range attraction between large bipolarons. With attractive interactions between large bipolarons, they can condense into a liquid phase. This liquid is a quantum liquid if the ground state of the interacting large bipolarons is a fluid rather than a solid. This quantum liquid of charged bosons is analogous to the quantum liquid of neutral bosons envisioned for superfluid He. As such, the superconductivity of large bipolarons can be understood (or rationalized) in a similar manner to that employed in addressing the superfluidity of liquid He.

Introduction

This article begins by describing electron–lattice interactions of ionic solids. A long-range electron–lattice interaction results from the dependence of the Coulombic potential energy of a carrier on the positions of the solid's ions. [1] Short-range electron–lattice interactions reflect the sensitivity of the energy of a carrier's local state (e.g., bonding or antibonding state) to the positions of nearby atoms [2,3].

The notions of self-trapping and bipolaron formation are then reviewed. Since a self-trapped carrier can only move when atoms move, the adiabatic approach is employed to discuss polaron and bipolaron formation.