SMALL-BIPOLARON HOPPING CONDUCTIVITY IN CHALCOGENIDE GLASSES

We applied a simplified microscopic two-site double-well model of smal l bipolarons (SB's) executing thermally enhanced adiabatic tunneling t o calculate the complex dynamic conductivity of chalcogenide glasses i n good quantitative agreement with the experimental data for As2Se3. I t was found that in these materials (i) the primary mechanism for the ac conductivity is SB hopping, which at low temperatures involves adia batic tunneling in the atomic subsystem; (ii) the intersite distance ( not the energy disparity) is the Principal source of the relaxation ti me dispersion; (iii) the stretched bonds of the size of 4.0+/-0.6 Angs trom with the concentration of the order of 10(21) cm(-3) are responsi ble for observed dielectric losses in a wide range of temperatures (0- 600 K) and frequencies (10(2)-10(8) Hz); (iv) the spatial distribution of the SB sites as well as of their occupation numbers are strongly a ffected by the pair Coulombic correlations. Both the temperature and t he frequency dependencies of the real part of the ac conductivity as w ell as of the dielectric losses' tangent have been discussed.