BIPOLARON AC CONDUCTIVITY IN AMORPHOUS-SEMICONDUCTORS AND DIELECTRICS

We have developed a theory of the alternating-current (ac) relaxation- type conductivity due to small bipolaron (SB) hopping in amorphous sem iconductors and insulators that possess deep centers of the dangling-b ond type (D centers) with a negative two-electron correlation energy U (eff). Unlike small polarons, SB's were treated as essentially three-l evel systems in the framework of a two-site approximation. To calculat e, both numerically and analytically, the real part sigma1 is-proporti onal-to omega(s)T(n) of the ac hopping conductivity for different temp eratures T in a wide range of audio and low radio frequencies omega, t he dynamic polarizability, and the SB hopping rates for dangling bond pairs have been determined. When the electron tunneling integral corre sponding to the smallest intersite separations is greater than the dou bled polaron shift, both the polarizability and the hopping rate stron gly depend upon the shape and parameters of the ground-state adiabatic potential of a small-size pair of strongly interacting D centers. Thi s intimate pair can be viewed as a stretched or weakened bond. A class ification of possible regimes of relaxation-type SB hopping (adiabatic and nonadiabatic, as well as tunnel and activation) has been proposed . Each of these corresponds to a specific temperature dependence of th e exponents s and n. A comparison to experimental data on ac losses in chalcogenide glasses and a-SiO2 has been made. It demonstrates that, in these materials, a tunneling adiabatic (TA) regime of SB hopping wi th s almost-equal-to 1 and n much less than 1 has been observed at low temperatures provided that there are (i) strong Coulombic correlation s in the intersite occupations and (ii) strong scattering of one-elect ron energies. As the temperature is increased, the TA regime seems to be replaced by the activation adiabatic regime with s < 1 being a decr easing function and n > 1 being an increasing function of T. It was fo und that in the negative-U(eff) amorphous systems the stretched bonds of size of 3.5-4.5 angstrom can be responsible for ac losses within th e wide frequency range from 10(2) to 10(8) Hz.