Theory of resonant X-ray emission spectra for high Tc cuprates

Resonant X-ray emission spectroscopy (RXES) is a powerful tool in the study of electronic states of various materials. In this paper, we report two recent topics of RXES for high Tc related cuprates: Cu Kα RXES of La2CuO4 and Cu 1s → 4p → 1s RXES of one-dimensional cuprates. We first show from both experimental and theoretical studies that the Cu Kα RXES in La2CuO4 is extremely sensitive to fine structures in the pre-edge region of the Cu 1s X-ray absorption. Calculations combining the Cu 4p density of states obtained by ab initio method with a many-body cluster model including a single Cu site show remarkable agreement with experiment. In particular, we show that an extremely weak off-site Cu 1s → 3d dipole transition-mediated by Cu 4p-O 2p and O 2p-Cu 3d hybridizations can also be clearly detected by the Cu Kα RXES. Then we give theoretical calculations of the Cu 1s → 4p → 1s RXES spectra for one-dimensional Cu6O18 cluster model. The results indicate that the RXES spectra are sensitive to the energy dispersion of a pair excitation of an upper-Hubbard band electron and a Zhang-Rice-singlet band hole, the excitation of local antibonding state between 3d9 and 3d10L configurations, and the crystal field excitation. The role of the Cu 3d orbital degeneracy is also discussed.