Electronic Structure of Doped Lanthanum Cuprates Studied with Resonant Inelastic X-Ray Scattering

We report a comprehensive Cu $K$-edge RIXS investigation of $\rm La_{2-x}Sr_xCuO_4$(LSCO)for0$\leq$x$\leq$0.35,stripe-ordered$\rm La_{1.875}Ba_{0.125}CuO_4$(LBCO),and$\rm La_{2}Cu_{0.96}Ni_{0.04}O_4$ (LCNO) crystals. The RIXS spectra measured at three high-symmetry momentum transfer ($\mathbf{\text{q}}$) 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 $\mathbf{\text{q}}$=($\pi$, 0) corresponding to the zone boundary. As hole doping increased, the spectral weight transfer from high energies to low energies is nearly linear with \emph{x} at this $\mathbf{\text{q}}$. We interpret the peaks as interband transitions in the context of existing band models for this system, assigning them to Zhang-Rice band$\to$upper Hubbard band, lower-lying band$\to$upper Hubbard band, and lower-lying band$\to$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 around 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 K 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.