EVIDENCE OF LOCALIZED STATES IN THE OPTICAL GAP OF CUO

Optical investigation has been done on single crystalline samples of t he semiconductor cupric oxide (CuO). A structureless reflectivity has been measured below the gap of 10 500 cm-1 down to the energies corres ponding to phonon excitations (below 700 cm-1). Transmission measureme nts, at temperatures above and below the AFM transitions, nevertheless , reveal the existence of absorption bands in the same spectral region . A broad band has been detected centered at about 2000 cm-1, and narr ow structures between 800 and 1100 cm-1, which could be related to the presence of localized d levels inside the gap.