Origin of unexpected large Seebeck effect in SrTiO3: nonperturbative polaron study from ab initio cumulant expansion

The experimental unexpected large Seebeck coefficient in SrTiO3 (STO) cannot be reproduced theoretically by the conventional Bloch-Boltzmann transport with electron-phonon coupling calculated perturbatively, indicating a failure of Fermi liquid picture in STO and ill-treatment of polaronic states. Starting from many-body interaction picture, the polaronic states can be precisely described by the method of cumulant expansion of retarded Green's function. By applying Kubo-Greenwood method, we found that the mysterious Seebeck coefficient can be fully described by a combination of polarons and Fermi liquid. The coexistence of two types of quasi-particles are attributed to the multi-band nature of t2g orbitals. Polarons are formed from the one with heavier electronic effective mass, while the one with lighter effective mass is renormalized into Fermi liquid quasi-particles. Our method provides a practical way to study the effect of strong electron-phonon coupling on transport properties from first principles.