Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper-oxide superconductors

TL;DR

This study reveals a strong anomaly in the Cu-O bond-stretching phonon in cuprate superconductors, linked to charge inhomogeneity, suggesting electron-phonon coupling plays an indirect role in high-temperature superconductivity.

Contribution

It provides experimental evidence of a giant electron-phonon anomaly in cuprates associated with charge order, challenging conventional theory and highlighting the importance of charge inhomogeneity.

Findings

Strong phonon anomaly observed in cuprates with charge order.

Anomaly correlates with spatial charge inhomogeneity and stripe order.

Absent in non-superconducting and overdoped samples.

Abstract

The attempt to understand cuprate superconductors is complicated by the presence of multiple strong interactions. While many believe that antiferromagnetism is important for the superconductivity, there has been revived interest in the role of electron-lattice coupling. The recently studied conventional superconductor MgB2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon), that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bond-stretching phonon in the cuprate superconductors La2-xSrxCuO4 (with x=0.07, 0.15). This is in contrast to conventional theory, which does not predict such behavior. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order. It occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron-phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron-phonon coupling may be important to our understanding of superconductivity, although its contribution to the mechanism is likely to be indirect.