Gapped collective charge excitations and interlayer hopping in cuprate superconductors

TL;DR

This study uses RIXS to investigate charge excitations in electron-doped cuprates, revealing a plasmon gap linked to interlayer hopping, highlighting the three-dimensional nature of charge dynamics in layered cuprates.

Contribution

It demonstrates the presence of a plasmon gap in cuprates and connects this gap to interlayer hopping, offering a new way to measure $t_z$ and understand charge dynamics.

Findings

Detection of a ~120 meV plasmon gap at zone center

Plasmon dispersion matches layered $t$-$J$-$V$ model calculations

Plasmon gap size correlates with interlayer hopping $t_z$

Abstract

We use resonant inelastic x-ray scattering (RIXS) to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr$_{0.9}$La$_{0.1}$CuO$_2$ (SLCO). We detect a plasmon gap of $\sim$~120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in SLCO are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered $t$-$J$-$V$ model calculations. A similar analysis performed on recent RIXS data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping $t_z$. Our work signifies the three-dimensionality of the charge dynamics in layered cuprates and provides a new method to determine $t_z$.