Probing Finite-Momentum Charge Collective Modes in Electron Doped SrTiO₃

Electron doped strontium titanate, SrTiO₃, becomes a bulk superconductor at carrier densities as low as 5.5×10¹⁷ cm^-3. The pairing mechanism in this dilute material, which is unconventional and widely debated, has recently been explained in terms of hybridization between the longitudinal optic phonons and the electronic plasmon collective modes. Using momentum-resolved inelastic electron scattering (M-EELS), we observed sub-200 meV acoustic and optic phonons, which have previously been implicated in the superconducting pairing, and a valence plasmon from the free carriers. For a carrier density of 2.137×10²⁰ cm^-3, the plasmon blueshifts with decreasing temperature, while its width and dispersion deviate from RPA predictions. In this talk, I will discuss evidence for hybridization of these modes for different carrier densities.