Electronic structure of doped lanthanum cuprates studied with resonant inelastic x-ray scattering

We report a comprehensive Cu K-edge resonant inelastic x-ray scattering (RIXS) investigation of La2-xSrxCuO4 (LSCO) for 0 <= x <= 0.35, stripe-ordered La1.875Ba0.125CuO4 (LBCO), and La2Cu0.96Ni0.04O4 (LCNO) crystals. The RIXS spectra measured at three high-symmetry momentum-transfer (q) positions are compared as a function of doping and for the different dopants. The spectra in the energy range 1-6 eV can be described with three broad peaks, which evolve systematically with increased doping. The most systematic trend was observed for q = (pi,0) corresponding to the zone boundary. As hole doping increased, the spectral weight transfer from high energies to low energies is nearly linear with x at this q. We interpret the peaks as interband transitions in the context of existing band models for this system, assigning them to Zhang-Rice band -> upper Hubbard band, lower-lying band -> upper Hubbard band, and lower-lying band -> Zhang-Rice band transitions. The spectrum of stripe-ordered LBCO was also measured, and found to be identical to the correspondingly doped LSCO, except for a relative enhancement of the near-infrared peak intensity at similar to 1.5-1.7 eV. The temperature dependence of this near-infrared peak in LBCO was more pronounced than for other parts of the spectrum, continuously decreasing in intensity as the temperature was raised from 25 to 300 K. Finally, we find that 4% Ni substitution in the Cu site has a similar effect on the spectra as does Sr substitution in the La site.