Evidence for spin-orbit-assisted electron-phonon coupling in metallic SrTiO₃

Strontium titanate is a prominent quantum material that has been studied for over six decades. With an almost ideal perovskite structure, it exhibits strong ferroelectric fluctuations and superconductivity at ultra-low charge carrier densities. While SrTiO₃ was the first oxide superconductor to be discovered, the microscopic mechanism behind its superconductivity remains debated. It has recently been proposed that a dynamic Rashba interaction between soft polar phonon modes and conducting electrons plays a pivotal role [1]. Such coupling would lead to hybridization of phonon, spin-flip, and plasmon modes in applied magnetic fields [2]. To test this prediction, we have performed an inelastic cold-neutron scattering experiment in a sample with a carrier concentration of ~10¹⁹ cm^-3. Measurements of the soft zone-center polar phonon show evidence of a splitting of the lowest phonon branch in a magnetic field, consistent with the predicted spin-phonon hybridization due to Rashba-like coupling.

[1] M. N. Gastiasoro et al., Phys. Rev. B 105, 224503 (2022)

[2] A. Kumar et al., Phys. Rev. B 105, 125142 (2022)