Evolution of plasmon excitations across the phase diagram of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$

TL;DR

This study investigates how plasmon excitations in the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$ evolve with doping and temperature using resonant inelastic x-ray scattering, revealing saturation effects and theoretical-experimental discrepancies.

Contribution

It provides new experimental data on plasmon behavior across the doping phase diagram and compares these results with layered $t$-$J$-$V$ model calculations, highlighting areas of agreement and discrepancy.

Findings

Plasmon energy increases with doping in the underdoped regime.

Saturation of plasmon energy occurs above optimal doping ($x \,\gtrsim\, 0.16$).

Temperature has minimal effect on plasmon excitations.

Abstract

We use resonant inelastic x-ray scattering (RIXS) at the O $K$- and Cu $K$-edges to investigate the doping- and temperature dependence of low-energy plasmon excitations in La$_{2-x}$Sr$_{x}$CuO$_4$. We observe a monotonic increase of the energy scale of the plasmons with increasing doping $x$ in the underdoped regime, whereas a saturation occurs above optimal doping $x \gtrsim 0.16$ and persists at least up to $x = 0.4$. Furthermore, we find that the plasmon excitations show only a marginal temperature dependence, and possible effects due to the superconducting transition and the onset of strange metal behavior are either absent or below the detection limit of our experiment. Taking into account the strongly correlated character of the cuprates, we show that layered $t$-$J$-$V$ model calculations accurately capture the increase of the plasmon energy in the underdoped regime. However, the computed plasmon energy continues to increase even for doping levels above $x \gtrsim 0.16$, which is distinct from the experimentally observed saturation, and reaches a broad maximum around $x = 0.55$. We discuss whether possible lattice disorder in overdoped samples, a renormalization of the electronic correlation strength at high dopings, or an increasing relevance of non-planar Cu and O orbitals could be responsible for the discrepancy between experiment and theory for doping levels above $x = 0.16$.