Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${ \bf q}_{ \parallel}=(0,0)$ for various doping rates in electron-doped cuprates ${ \rm La_{ 2-x}Ce_xCuO_4}$. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered $t$-$J$ model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.
Categories: cond-mat.supr-con cond-mat.str-el