New Fine Structures near the Optical Absorption Edge of CuO at Low Temperatures

By using CuO crystals prepared with the floating zone technique, we have observed for the first time an emergence of new fine structures with an energy splitting of 5.86 meV in the optical absorption spectra κ( T ,ν) near the fundamental absorption edge, E G ∼1.60 eV, below 90 K. These results are ascribable either to [1] the direct allowed n = 1 exciton transitions below 90 K at the absorption edge, or [2] the transition due to magnetically ordered localized states such as spin-canted weak ferromagnetic states with a possible splitting due to the exchange interaction E ex =5.86 meV in the hybridized Cu(3d)O(2p) valence electronic system of antiferromagnetic semiconductor CuO.