Researchers report that a single strongly correlated model can reproduce low-energy, acousticlike plasmons seen by x-ray experiments in the cuprate superconductor La2−xSrxCuO4 across a wide range of hole concentrations. Plasmons are collective oscillations of the electron charge. The paper shows these charge waves appear with very similar energy and momentum behavior from underdoped to heavily overdoped samples, despite the many different electronic phases that the material passes through.

The team collected existing resonant inelastic x-ray scattering (RIXS) measurements on La2−xSrxCuO4 spanning hole dopings δ = 0.05 to 0.40 and compared them with calculations from a layered t-J-V model. This model extends the usual two-dimensional t-J model by adding the long-range Coulomb interaction V and a small hopping between layers. The authors compute the charge–charge response (the quantity RIXS is sensitive to) and extract the plasmon energies to compare directly with experiment.

At a technical level, the model enforces the no-double-occupancy rule that is important for strongly correlated electrons using a path-integral method and a large-N expansion. The authors used a single microscopic parameter set that was previously fixed for an optimally doped sample (δ = 0.16). Key numbers given are t′/t = −0.2, tz/t = 0.01, Vc/t = 31, α = 3.5, Γ/t = 0.1, J/t = 0.3, with t/2 = 0.35 eV. For every comparison they changed only the hole doping δ, the temperature T, and the experimental momentum transfer. With no further tuning, the calculated plasmon dispersions match most of the available RIXS data.

This result matters because many other measured properties of cuprates change strongly across the phase diagram — for example the pseudogap, charge and spin order, and superconductivity. The success of one fixed parameter set suggests that the collective plasmon mode is a robust feature set mainly by strong correlations and the layered Coulomb interaction, and that strong correlations remain important even in the heavily overdoped regime. In other words, the plasmon seems only weakly affected by the phase-specific electronic phenomena that distinguish different parts of the phase diagram.