Photogeneration of charged ferroelectric domains in quantum dielectric SrTiO 3

Abstract

In connection with the recent experimental discoveries on photo-enhancements of the electronic conductivity and the quasi-static electric susceptibility in SrTiO 3, we theoretically study a photogeneration mechanism of charged ferroelectric domains in this perovskite-type quantum dielectric. The photogenerated electron, being quite itinerant in the 3d band of Ti 4+, is assumed to couple weakly but quadratically with soft-anharmonic T 1u phonons in this quantum dielectric. This photogenerated electron is also assumed to couple strongly but linearly with the breathing(A 1g) type high-energy phonons. Using a discrete model, we will show that these two types of electron–phonon couplings result in two types of polarons, a “super-para-electric large polaron” with a quasi-global parity violation, and an “off-center type self-trapped polaron” with only a local parity violation. We will also show that this super-para-electric large polaron is nothing else but a singly charged (e −) and conductive ferroelectric domain with a quasi-macroscopic size. This large polaron is also shown to aggregate with other same polarons, resulting in large bipolarons and their clusters.