Observation of a 500meV Collective Mode in La$_{2-x}$Sr$_x$CuO$_4$ and Nd$_2$CuO$_4$

TL;DR

This study reports the discovery of a 500 meV collective excitation in La$_{2-x}$Sr$_x$CuO$_4$ using resonant inelastic x-ray scattering, revealing its dependence on doping and polarization, and its presence in Nd$_2$CuO$_4$.

Contribution

The paper identifies a new high-energy collective mode in cuprates and explores its behavior across different doping levels and related compounds, providing insights into their excitation spectra.

Findings

The 500 meV mode peaks at ($ackslashpi$,0) in reciprocal space.

The mode is suppressed with increased Sr doping and absent at x=0.17.

The mode is polarization-dependent and also observed in Nd$_2$CuO$_4$.

Abstract

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La$_{2-x}$Sr$_x$CuO$_4$ at 500 meV. The mode is peaked around the ($\pi$,0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at $x$=0.17, where it is replaced by a continuum of excitations. The peak is only observed when the incident x-ray polarization is normal to the CuO planes and is also present in Nd$_2$CuO$_4$. We suggest possible explanations for this excitation.