The Role of Hybridization Between Transverse Optical and Acoustic Phonons on Superconductivity in SrTiO3

We revisit the elusive nature of superconductivity in SrTiO3 in light of recent experiments and first-principles calculations that reveal significant hybridization (mixing) between low-energy transverse optical (TO) and acoustic (TA) phonons. The transverse phonon hybridization, which originates from the flexoelectric coupling between polarization and the strain gradient deformation, can induce the softening of the TA phonon branch at a non-zero wave vector. In the strongly hybridized limit, this softening triggers a novel instability in the system toward a modulated strained phase, alongside the well-studied proximity to ferroelectric order due to the softening of the TO phonons. Starting from a derived microscopic model that accounts for the coupling of electrons to hybridized transverse phonons, we investigate the pairing instabilities of the system near the modulated strained phase. We derive the effective pairing interaction and superconducting gap equation, solving for the critical temperature in the weak-coupling (BCS) regime. Finally, we discuss the role of uniaxial strain as a means to tune superconductivity in the system.