Evidence for a charge collective mode associated with superconductivity in copper oxides from neutron and x-ray scattering measurements of La$_{2-x}$Sr$_x$CuO$_4$

TL;DR

This study provides evidence that a collective charge mode, linked to phonon anomalies, plays a significant role in the superconductivity of copper oxides, as shown by neutron and x-ray scattering measurements.

Contribution

It demonstrates that the phonon anomaly in La$_{2-x}$Sr$_x$CuO$_4$ is due to collective charge excitations, not electron-hole interactions, and correlates with the superconducting dome.

Findings

Phonon anomalies are doping-dependent and distinct from ARPES kinks.

Charge excitations associated with phonon anomalies follow the superconducting dome.

Results suggest collective charge modes are important for superconductivity.

Abstract

In superconducting copper oxides some Cu-O bond-stretching phonons around 70meV show anomalous giant softening and broadening of electronic origin and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped (x=0.05) and overdoped (x=0.20,0.25) La$_{2-x}$Sr$_x$CuO$_4$ by inelastic neutron and x-ray scattering with high resolution. Combining these and previously published data, we found that doping-dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e. the two phenomena are not connected. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity.