Far-infrared study of electron-phonon coupling in metallic strontium titanate

Recently it was proposed that spin-orbit assisted electron-phonon coupling could be strong enough to explain superconducting pairing in metallic strontium titanate, SrTiO₃ (STO). This coupling can be represented as a dynamic Rashba spin-orbit interaction and results in a complex hybridization between plasmon, phonon and electron spin-flip modes in an applied magnetic field. We studied samples of lightly doped STO using a quasi-optical far-infrared spectrometer, which allowed for continuous measurements of absorbed power in the sub-terahertz frequency range. Measurements were made with and without a magnetic field. Without magnetic field, we observed two soft infrared TO phonon modes, due to crystal-field splitting in the STO tetragonal phase, and detected changes in their frequencies and weights with increasing field. Moreover, we found that a phonon-like signal appears at frequencies consistent with the screened plasmon energy. These results qualitatively support the aforementioned hybridization and lay the foundation to determine the coupling strength and resolve the origin of superconductivity in STO.