Plasmon excitations in layered high- Tc cuprates

Abstract

Motivated by the recent resonant inelastic x-ray scattering (RIXS) experiment for the electron-doped cuprates Nd2-xCexCuO4 with x≈0.15, we compute the density-density correlation function in the t-J model on a square lattice by including interlayer hopping and the long-range Coulomb interaction. We find that collective charge excitations are realized not inside the particle-hole continuum, but above the continuum as plasmons. The plasmon mode has a rather flat dispersion near the in-plane momentum q=(0,0) with a typical excitation energy of the order of the intralayer hopping t when the out-of-plane momentum qz is zero. However, when qz becomes finite, the plasmon dispersion changes drastically near q=(0,0), leading to a strong dispersive feature with an excitation gap scaled by the interlayer hopping tz. We discuss the mode recently observed by RIXS near q=(0,0) in terms of the plasmon mode with a finite qz.