Multiple nodeless superconducting gaps in optimally doped SrTi1-x Nbx O3

We present a study of thermal conductivity in superconducting SrTi1-xNbxO3, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive Tc. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk Tc, following a Bardeen-Rickayzen-Tewordt behavior, which is expected for thermal transport by Bogoliubov excitations. The absence of a T-linear term at very low temperatures rules out the presence of nodal quasiparticles. On the other hand, the field dependence of thermal conductivity points to the existence of at least two distinct superconducting gaps. We conclude that optimally doped strontium titanate is a multigap nodeless superconductor.