RAMAN STUDIES OF CHARGE-TRANSFER INSULATING CUPRATES

We have studied the electronic interband excitations in insulating cuprates using both resonant and ordinary Raman techniques. The resonant behaviors of two-magnon and two-LO(E(u)) phonon Raman scattering in YBa2Cu3O6, Pr2CuO4 and Gd2CuO4 with the charge-transfer band (1.7-2.7 eV) are found to be dramatically different. The phonon-pair scattering resonates strongly with the strong 1.7 eV peak at the optical absorption edge. The magnon-pair Raman scattering vanishes there, resonating at 2.7 eV, indicating perhaps that this denotes the strongest singlet interband transition. High energy Raman scattering allowed us to reveal, for the first time, dipole forbidden interband excitations. A well-defined peak at 1.5 eV for YBa2Cu3O6 and Gd2CuO4 has an unusual pseudovector A2g symmetry, indicating its magnetic character.