Measured with meV-Resolution EELS*

The emergence of superconductivity in low carrier solids does not follow the behavior of conventional BCS superconductivity. Niobium doped strontium titanate, SrTi_1-xNb_xO₃, which becomes a superconductor at densities as low as 5.5×10¹⁷ cm^-3 [1], is theorized to come about due to the hybridization of the longitudinal optic phonons with the plasmons. In this talk, I will present measurements of the collective charge excitations of SrTi_1-xNb_xO₃ using momentum-resolved inelastic electron scattering (M-EELS). We observed a variety of acoustic and optic phonons in the range 0-200 meV that have been implicated in the superconducting pairing in this material. In addition, we observed an electronic mode at 25 meV that we identify as a valence plasmon from the Nb free carriers. I will discuss evidence for hybridization of these modes for different Nb concentration, x.

[1] X. Lin, Z. Zhu, B. Fauqué, and K. Behnia, Phys. Rev. X 3, 021002 (2013).