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 T-c related cuprates: Cu K alpha 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 K alpha RXES in La2CuO4 is extremely sensitive to fine structures in the pre-edge region of the Cu Is 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 K alpha 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 3d(9) and 3d(10)L configurations, and the crystal field excitation. The role of the Cu 3d orbital degeneracy is also discussed. (c) 2006 Elsevier B.V. All rights reserved.