Novel Raman-active electronic excitations near the charge-transfer gap in insulating cuprates

We have observed electric-dipole-forbidden electronic excitations with ${\mathit{A}}_{2\mathit{g}}$, ${\mathit{B}}_{1\mathit{g}}$, and ${\mathit{A}}_{1\mathit{g}}$ symmetries at energies near the charge-transfer gap in insulating cuprates, using large-shift Raman techniques. The strongest feature, with pseudovector ${\mathit{A}}_{2\mathit{g}}$ 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. ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ spectra at different doping levels show that all the new features are suppressed in metallic phases. We propose two possible interpretations: internal ${\mathit{d}}^{9\mathrm{-}}$ ${\mathit{d}}^9$ and charge-transfer ${\mathit{d}}^{9\mathrm{-}}$ ${\mathit{d}}^{10}$L excitations.