Origin and roles of a strong electron-phonon interaction in cuprate oxide superconductors

TL;DR

This paper explores the origin of strong electron-phonon interactions in cuprate superconductors, linking phonon softening to magnetic fluctuations and clarifying their limited role in superconductivity.

Contribution

It reveals that electron-phonon interactions stem from superexchange modulation and explains phonon softening phenomena without attributing a primary role to them in superconductivity.

Findings

Phonon softening occurs around specific wave vectors due to superexchange modulation.

Stripe and checker-board states' density of states can be explained by SDW second-harmonic effects.

Electron-phonon interaction plays a minor role in cuprate superconductivity.

Abstract

A strong electron-phonon interaction arises from the modulation of the superexchange interaction by phonons. As is studied in Phys. Rev. B 70, 184514 (2004), Cu-O bond stretching modes can be soft around (pm pi/a, 0) and (0, pm pi/a), with a the lattice constant of CuO_2 planes. In the critical region of SDW, where antiferromagnetic spin fluctuations are developed around nesting wave numbers Q of the Fermi surface, the stretching modes can also be soft around 2Q. Almost symmetric energy dependences of the 2Q component of the density of states, which are observed in the so called stripe and checker-board states, cannot be explained by CDW with 2Q following the complete softening of the 2Q modes, but they can be explained by a second-harmonic effect of SDW with Q. The strong electron-phonon interaction can play no or only a minor role in the occurrence of superconductivity.