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

TL;DR

This paper explains the large Seebeck effect in SrTiO3 by combining polaron and Fermi liquid models using a nonperturbative ab initio cumulant expansion approach, revealing the coexistence of two quasi-particle types.

Contribution

It introduces a first-principles cumulant expansion method to accurately describe polaronic states and transport in SrTiO3, surpassing conventional perturbative approaches.

Findings

Large Seebeck coefficient explained by polaron and Fermi liquid coexistence.

Polaron formation linked to heavier effective mass orbitals.

Method provides a new way to study strong electron-phonon coupling effects.

Abstract

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.