Coupling between electrons and hybridized transverse optical-acoustic phonons in SrTiO3

The emergence of dilute superconductivity that cannot be accounted for by the standard electron-phonon mechanism remains an enduring mystery in the extensively studied quantum paraelectric SrTiO3. Besides the existence of a soft transverse optical (TO) mode associated with ferroelectric order, recent experiments revealed a softening of the transverse acoustic (TA) phonon branch at a non-zero wave-vector, indicative of an instability towards a modulated strained phase. Such a softening is attributed to the hybridization between the low-energy TO and TA phonons via the flexoelectric effect. Motivated by this finding, we combine first-principles calculations and phenomenological modeling to investigate the electron-transverse phonon coupling in SrTiO3. Focusing on the low-energy hybridized TO-TA phonons, we determine both the low-temperature thermodynamic properties and the pairing instabilities of the electron-phonon coupled system. We also discuss the implications of our results to other paraelectrics, such as KTaO3.