MOMENTUM-DEPENDENT CHARGE-TRANSFER EXCITATIONS IN SR2CUO2CL2 ANGLE-RESOLVED ELECTRON-ENERGY-LOSS SPECTROSCOPY

Electron-hole pair excitations in the insulating cuprates Sr2CuO2Cl2 w ere investigated by angle-resolved electron energy loss spectroscopy. The optically allowed and optically forbidden transitions were observe d to be strongly anisotropic in the Cu-O-2 plane. The former show a la rge energy dispersion similar to 1.5 eV along [110], and the latter ap pear at a higher energy position (similar to 4.5 eV) only along [100], but not along [110]. We interpret these results as transitions involv ing excitons. A small exciton model is examined to explain both the ob served features.