Photogeneration of superparaelectric large polarons in dielectrics with soft anharmonic T1u phonons -: art. no. 174111

In connection with the recent experiments on photoenhancements of the electronic conductivity and the quasistatic electric susceptibility of SrTiO3, we theoretically study global versus local parity violation of a photogenerated electron in this dielectric. The photogenerated electron, being quite itinerant in the 3d band of Ti, is assumed to couple weakly but quadratically with soft-anharmonic T-1u phonons in this dielectric. The electron is also assumed to couple strongly but linearly to the breathing (A(1g)) type high energy phonons. We will show that these two types of electron-phonon (e-p) couplings result in two types of polarons, a superparaelectric large polaron with a quasiglobal parity violation, and an off-center type self-trapped polaron with only a local parity violation. These two states are shown to be separated by an adiabatic potential barrier, if these e-p couplings are short in their force ranges. Without the T-1u phonon, these two states reduce to the well-known large polaron and the self-trapped one, both of which have even parities. We will also show that this superparaelectric large polaron enhances both the electronic conductivity and the quasistatic electric susceptibility, in qualitative agreements with the experiments.