Temperature Dependence of the Dielectric Response and Critical Point Energies of Bi1.85Gd0.15Te3.

The complex dielectric function and band-edge critical point structures of Bi1.85Gd0.15Te3 are reported for temperatures from 28 to 300 K and energies from 0.74 to 6 eV, obtained on bulk Bi1.85Gd0.15Te3 by rotating-compensator spectroscopic ellipsometry. The critical point (CP) energies are determined using numerically calculated second energy derivatives of the data. At low temperature, eight CP structures are identified, while only four CPs are observed at room temperature. As temperature decreases, we also observe blue shifts and significantly enhanced CP structures relative to those obtained at room temperature. The temperature dependence of the CPs are determined by fitting the data to the temperature coefficient and a phenomenological expression that contains the Bose-Einstein statistical factor.