Momentum-Dependent Charge Transfer Excitations in ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_2$${\mathrm{Cl}}_2$ Angle-Resolved Electron Energy Loss Spectroscopy

Electron-hole pair excitations in the insulating cuprates ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_2$ ${\mathrm{Cl}}_2$ were investigated by angle-resolved electron energy loss spectroscopy. The optically allowed and optically forbidden transitions were observed to be strongly anisotropic in the Cu-${\mathrm{O}}_2$ plane. The former show a large energy dispersion ∼1.5 eV along [110], and the latter appear at a higher energy position (∼4.5 eV) only along [100], but not along [110]. We interpret these results as transitions involving excitons. A small exciton model is examined to explain both the observed features.