Multiple nodeless superconducting gaps in optimally-doped   SrTi$_{1-x}$Nb$_{x}$O$_{3}$
  We present the first study of thermal conductivity in superconducting SrTi$_{1-x}$Nb$_{x}$O$_{3}$, 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 T$_{c}$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk T$_{c}$, following a Bardeen-Rickayzen-Tewordt (BRT) behavior, expected for thermal transport by Bogoliubov excitations. Absence of a T-linear term at very low temperatures rules out the presence of nodal quasi-particles. 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. 
Xiao Lin, Adrien Gourgout, German Bridoux, Francois Jomard, Alexandre   Pourret, Benoit Fauque, Dai Aoki and Kamran Behnia
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el