Evolution of plasmon excitations across the phase diagram of the cuprate superconductor La2−xSrxCuO4

We use resonant inelastic x-ray scattering (RIXS) at the O $K$- and Cu\n$K$-edges to investigate the doping- and temperature dependence of low-energy\nplasmon excitations in La$_{2-x}$Sr$_{x}$CuO$_4$. We observe a monotonic\nincrease of the energy scale of the plasmons with increasing doping $x$ in the\nunderdoped regime, whereas a saturation occurs above optimal doping $x \\gtrsim\n0.16$ and persists at least up to $x = 0.4$. Furthermore, we find that the\nplasmon excitations show only a marginal temperature dependence, and possible\neffects due to the superconducting transition and the onset of strange metal\nbehavior are either absent or below the detection limit of our experiment.\nTaking into account the strongly correlated character of the cuprates, we show\nthat layered $t$-$J$-$V$ model calculations accurately capture the increase of\nthe plasmon energy in the underdoped regime. However, the computed plasmon\nenergy continues to increase even for doping levels above $x \\gtrsim 0.16$,\nwhich is distinct from the experimentally observed saturation, and reaches a\nbroad maximum around $x = 0.55$. We discuss whether possible lattice disorder\nin overdoped samples, a renormalization of the electronic correlation strength\nat high dopings, or an increasing relevance of non-planar Cu and O orbitals\ncould be responsible for the discrepancy between experiment and theory for\ndoping levels above $x = 0.16$.\n