Photo-genaration of super-para-electric large polaron

结合近年来关于SrTiO_3中电导率和准静态电磁化率的光增强实验发现，我们从理论上研究了这种钙钛矿型量子介质中带电铁电畴的光产生机制。光生电子在Ti^<4+>的三维带中是相当流动的，假设在该量子介质中与软非谐波T_<1u>声子弱但二次耦合。这个光生电子也被假定与呼吸（A_<1g>）型高能声子有强而线性的耦合。使用离散模型，我们将证明这两种类型的电子-声子耦合导致两种类型的极化子，具有准全局宇称破坏的“超对电大极化子”和仅具有局部宇称破坏的“离中心型自困极化子”。我们还将证明这个超对电大极化子只不过是一个具有准宏观尺寸的单电荷（e^-）和导电的铁电畴。这个大极化子也显示与其他相同的极化子聚集，产生大的双极化子及其团簇。

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.