NOVEL RAMAN-ACTIVE ELECTRONIC EXCITATIONS NEAR THE CHARGE-TRANSFER GAP IN INSULATING CUPRATES

We have observed electric-dipole-forbidden electronic excitations with A2g, B1g, and A1g symmetries at energies near the charge-transfer gap in insulating cuprates, using large-shift Raman techniques. The strongest feature, with pseudovector A2g symmetry, occurs in insulating cuprates just below the main optical absorption peak, varying from 0.15 to 0.2 eV below according to the Cu coordination number. YBa2Cu3O6+x spectra at different doping levels show that all the new features are suppressed in metallic phases. We propose two possible interpretations: internal d9-d9 and charge-transfer d9-d10L excitations.