Theory of Cu 2pX-Ray Emission Spectroscopyin CuO and La2CuO4

Soft X-ray emission spectroscopy due to the Cu 3 d →2 p electronic transition (Cu 2 p -XES) is one of the powerful tools for investigating the local nature of the Cu 3 d state in high- T c superconductors. We theoretically investigate Cu 2 p -XES for CuO and La 2 CuO 4 based upon the impurity Anderson model and the formula of the second order optical process. Justification for this model is given by that available experimental data of Cu 2 p -XES, 2 p -XPS, 2 p -XAS and 3 d -XPS are satisfactorily reproduced in a unified way. According to Cu 2 p -XES experiments so far made, a shoulder structure appears on the high energy side of the main peak in La 2 CuO 4 and also in high- T c superconductors, but it is absent in CuO. It is shown that the appearance of the shoulder structure originates from the stronger hybridization for the a 1 g symmetry states in La 2 CuO 4 than in CuO due to the existence of the apical oxygen.