Single-gap superconductivity and dome of superfluid density in Nb-doped SrTiO3

Abstract

SrTiO${}_3$ exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature \mbox{$T_{\mathrm{c}}\approx 0.4$~K}. Using microwave stripline resonators at frequencies from 2 to 23~GHz and temperatures down to 0.02~K, we probe the low-energy optical response of superconducting SrTiO${}_3$ with charge carrier concentration from 0.3 to $2.2\times 10^{20}$~cm${}^{-3}$, covering the majority of the superconducting dome. We find single-gap electrodynamics even though several electronic bands are superconducting. This is explained by a single energy gap $2\Delta$ due to gap homogenization over the Fermi surface consistent with the low level of defect scattering in Nb-doped SrTiO${}_3$. Furthermore, we determine $T_{\mathrm{c}}$, $2\Delta$, and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.

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