Unconventional superconducting pairing by conventional phonons

TL;DR

This paper challenges the traditional view that phonons only produce conventional s-wave pairing in superconductors, showing that anisotropy and strong coupling can lead to unconventional pairing symmetries like d-wave.

Contribution

It demonstrates that phonons can induce unconventional superconducting pairing, including d-wave symmetry, due to anisotropy and strong coupling effects, revising the understanding of pairing mechanisms.

Findings

Anisotropy of sound velocity leads to non-local electron attraction.

Quantum phase transition from d-wave to s-wave with increasing carriers.

Conventional phonons can cause d-wave pairing in cuprates.

Abstract

The common wisdom that the phonon mechanism of electron pairing in the weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors leads to conventional s-wave Cooper pairs is revised. An inevitable anisotropy of sound velocity in crystals makes the phonon-mediated attraction of electrons non-local in space providing unconventional Cooper pairs with a nonzero orbital momentum in a wide range of electron densities. As a result of this anisotropy quasi-two dimensional charge carriers undergo a quantum phase transition from an unconventional d-wave superconducting state to a conventional s-wave superconductor with more carriers per unit cell. In the opposite strong-coupling regime rotational symmetry breaking appears as a result of a reduced Coulomb repulsion between unconventional bipolarons dismissing thereby some constraints on unconventional pairing in the Bose-Einstein condensation (BEC) limit. The conventional phonons, and not superexchange, are shown to be responsible for the d-wave symmetry of cuprate superconductors, where the on-site Coulomb repulsion is large.