Giant phonon anomalies and central peak due to charge density wave formation in YBa$_2$Cu$_3$O$_{6.6}$

TL;DR

This study uses high-resolution inelastic x-ray scattering to investigate phonon anomalies and charge density wave formation in underdoped YBa2Cu3O6.6, revealing strong electron-phonon interactions that influence electronic spectra but do not drive superconductivity.

Contribution

It provides direct experimental evidence of phonon anomalies and CDW nanodomains in YBa2Cu3O6.6, elucidating their roles in high-temperature cuprate superconductors.

Findings

Large superconductivity-induced phonon lineshape renormalizations near CDW wavevector

Observation of a quasi-elastic central peak indicating CDW nanodomains

Electron-phonon interaction influences electronic spectra but does not drive pairing

Abstract

The electron-phonon interaction is a major factor influencing the competition between collective instabilities in correlated-electron materials, but its role in driving high-temperature superconductivity in the cuprates remains poorly understood. We have used high-resolution inelastic x-ray scattering to monitor low-energy phonons in YBa$_2$Cu$_3$O$_{6.6}$ (superconducting $\bf T_c = 61$ K), which is close to a charge density wave (CDW) instability. Phonons in a narrow range of momentum space around the CDW ordering vector exhibit extremely large superconductivity-induced lineshape renormalizations. These results imply that the electron-phonon interaction has sufficient strength to generate various anomalies in electronic spectra, but does not contribute significantly to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW nanodomains is observed in a wide temperature range above and below $\bf T_c$, suggesting that the gradual onset of a spatially inhomogeneous CDW domain state with decreasing temperature is a generic feature of the underdoped cuprates.