Crystal-field and Zhang-Rice-singlet excitations in CuGe1-xSixO3 (x=0, 0.05, and 0.1) from temperature-, angle-, and polarization-dependent resonant soft x-ray emission

The absence of a clear signature of the Zhang-Rice singlet in the photoelectron valence band spectra of copper germanate raised the question of the stability of this state, induced by one-electron removal processes, in large-gap systems. Following Okada and Kotani [Phys. Rev. B 63, 045103 (2001)], we have performed a study of the O 1s resonant x-ray emission for CuGe1-xSixO3 (x=0, 0.05, and 0.1) single crystals as a function of temperature and polarization. Our experiment shows that the temperature, angle, and polarization dependence of the inelastic scattering emission detected at a 3.6 eV Raman shift in the O 1s resonant x-ray spectra can be unambiguously identified with the Zhang-Rice-singlet excitation.