Doping Dependence of Electronic Excitations in La$_{2-x}$Sr$_{x}$CuO$_{4}$ Observed by K-edge RIXS

Resonant inelastic xray scattering (RIXS) spectra of the cuprate La$_{2-x}$Sr$_{x}$CuO$_{4 }$are measured for single crystal samples with progressively larger value of x, ranging from undoped to the overdoped regime. As x is increased in the underdoped region, the lowest energy excitation above the charge transfer gap is sharply suppressed, but many of the general spectral features and overall spectral weight distribution above the gap do not appreciably change, and the broad intensity around the same energy of the exciton remains constant. As the sample becomes overdoped, a much more pronounced change of the spectral weight above the charge transfer gap occurs. An in-gap state at $\sim $1.8 eV in the undoped case, which shows no dispersion with momentum, broadens and shifts down in energy as the doping is increased. The amount of the observed shift is to within an order of magnitude of the change calculated from simple crystal field model. .