Doping dependence of charge-transfer excitations in La 2 − x Sr x CuO 4

We report a resonant inelastic x-ray scattering (RIXS) study of the doping dependence of charge-transfer excitations in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$. The momentum dependence of these charge excitations are studied over the whole Brillouin zone in underdoped $(x=0.05)$ and optimally doped $(x=0.17)$ samples, and compared with that of the undoped $(x=0)$ sample. We observe a large change in the RIXS spectra between the $x=0$ and $x=0.17$ samples, while the RIXS spectra of the $x=0.05$ sample are similar to that of the $x=0$ sample. The most prominent effect of doped holes on the charge excitation spectra is the appearance of a continuum of intensity, which exhibits strong momentum dependence below $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. For the $x=0.17$ sample, some of the spectral weight from the lowest-lying charge-transfer excitation of the undoped compound is transferred to the continuum intensity below the gap, in agreement with earlier optical studies. However, the higher-energy charge-transfer excitation carries significant spectral weight even for the $x=0.17$ sample. The doping dependence of the dispersion of this charge-transfer excitation is also discussed and compared with recent theoretical calculations.