Chiral phonons in metallic strontium titanate

Strontium titanate (SrTiO₃, STO) is a model quantum material with strong polar lattice dynamics and unconventional superconductivity at extremely low carrier densities. It has recently been proposed that a dynamic Rashba interaction between soft polar phonon modes and conduction electrons plays a pivotal role in its low-energy properties [1]. Such coupling is predicted to produce hybridization of phonon, spin-flip, and plasmon modes in external magnetic fields, giving rise to chiral phonons (lattice excitations carrying a definite helicity) and, therefore, to circular dichroism in their optical response [2]. In order to investigate this possibility, we use a polarization-resolved far-infrared spectroscopy setup capable of operation in high magnetic fields and at cryogenic temperatures. In oxygen-vacancy-doped STO, we observe a pronounced dependence of absorption on the polarization of the incident radiation, including a helicity-dependent circular dichroism when using circularly polarized light generated by a Fresnel rhomb [3]. These results demonstrate coupling between phonon and electronic degrees of freedom with a defined helicity, consistent with the emergence of chiral phonons driven by spin-orbit-assisted electron-phonon interaction.

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

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

[3] M. J. Lazarus, S. Petithomme, Microw. Opt. Techn. Let. 20, 353 (1999)