High-resolution RIXS measurement at O K-edge on the edge-shared chain cuprates, Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$

Quasi one dimensional copper oxides have been model systems in the field of correlated electron physics, because of rich phenomena exhibited in a relatively simple geometry. Its magnetic ground states, fluctuations, and excitations have been investigated extensively by theorists and experimentalists. Among the known quasi 1-D spin-chain compounds, Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$ is the only compound that can be hole-doped in a wide doping range, providing a unique opportunity to study the dynamics of hole in the quasi-1D environment. Here, we report ultrahigh resolution resonant inelastic soft x-ray scattering experiment at the O K-edge. With an energy resolution of $\sim $50 meV, we resolved rich charge excitations in the sub-eV range that has not been observed in the previous RIXS measurement on the same materials. In particular, we have resolved clear multi-phonon excitations near the elastic peak, suggesting a strong electron-phonon coupling in this quasi-1D system. Doping dependence of these excitations will also be demonstrated.