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) for 0$leq$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 (textbf{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 textbf{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 textbf{q}. We interpret the peaks as interband transitions in the context of existing band models for this system, assigning them to Zhang-Rice band$rightarrow$upper Hubbard band, lower-lying band$rightarrow$upper Hubbard band, and lower-lying band$rightarrow$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.

Magnetic nature of the 500 meV peak in $rm La_{2-x}Sr_{x}CuO_4$ observed with resonant inelastic x-ray scattering at the Cu $K$-edge

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at ${bf q}=(pi,0)$ in resonant inelastic x-ray scattering (RIXS) experiments on $rm La_2CuO_4$. The intensity of this peak persists above the Neel temperature (T$_{N}$=320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon $rm B_{1g}$ Raman peak in $rm La_2CuO_4$, and with suitable rescaling, agrees with the Raman peak shifts in $rm EuBa_2Cu_3O_6$ and $rm K_2NiF_4$. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

Comparison of Resonant Inelastic X-Ray Scattering Spectra and Dielectric Loss Functions in Copper Oxides

We report empirical comparisons of Cu K-edge indirect resonant inelastic x-ray scattering (RIXS) spectra, taken at the Brillouin zone center, with optical dielectric loss functions measured in a number of copper oxides. The RIXS data are obtained for Bi$_2$CuO$_4$, CuGeO$_3$, Sr$_2$Cu$_3$O$_4$Cl$_2$, La$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$, and analyzed by considering both incident and scattered photon resonances. An incident-energy-independent response function is then extracted. The dielectric loss functions, measured with spectroscopic ellipsometry, agree well with this RIXS response, especially in Bi$_2$CuO$_4$ and CuGeO$_3$.