Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb0.25Bi2Se3

Recently, the niobium (Nb) doped topological insulator Bi2Se3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi2Se3 layers, has been shown to exhibit both superconductivity with Tc≃3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth λeffT in the layered topological superconductor candidate Nb0.25Bi2Se3. The exponential temperature dependence of λeff-2(T) at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature Tc=2.9K and the gap to Tc ratio 2Δ/kBTc=3.95(19). We also reveal that the ratio Tc/λeff-2 is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field μSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of Nb0.25Bi2Se3.