Three-dimensional collective charge excitations in electron-doped cuprate superconductors

High temperature cuprate superconductors consist of stacked CuO2planes, with primarily two dimensional electronic band structures and magnetic excitations, while superconducting coherence is three dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which were found to be incoherent in the normal state, within the limited range of momenta accessible by optics. In this talk, I will discuss our results obtained by resonant inelastic x-ray scattering (RIXS) exploring the charge dynamics across all three dimensions of the Brillouin zone [1]. From polarization analysis, a pure charge character of a recently discovered collective mode in electron-doped cuprates is unveiled.This mode disperses along both the in- and, importantly, out-of-plane directions, revealing its three dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the CuO2plane distance rather than the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought acoustic plasmon, predicted for layered systems [2] and hypothesized to play a substantial role in mediatinghigh temperature superconductivity [3].