Single-Gap Superconductivity and Dome of Superfluid Density in Nb-Doped SrTiO3

SrTiO3 exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature T-c approximate to 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 SrTiO3 with a charge carrier concentration from 0.3 to 2.2 x 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 SrTiO3. Furthermore, we determine T-c, 2 Delta, and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.