Optical Study of Electronic Structure and Nonmetal-Metal Transition of Gd 2 S 3

Reflectivity spectra of a magnetic semiconductor material Gd 2 S 3 (=Gd 3- x V x S 4 , where V denotes vacancy and x =1/3) have been measured in the energy range from 2 meV to 40 eV at 9, 20, 77 and 300 K. From the comparison between the optical conductivity and the photoelectron spectra, the energy position and the shape of the occupied and the unoccupied electronic states were obtained. Above 77 K, this material showed a metallic reflectivity spectrum, but an energy gap of about 7 meV opened on the Fermi level below 20 K. The origin of the nonmetal-metal transition was concluded to originate from the creation of a magnetic polaron state.