Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb 0.25 Bi 2 Se 3

Recently, the niobium (Nb) doped topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layers, has been shown to exhibit both superconductivity with ${T}_{c}\ensuremath{\simeq}3$ K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth ${\ensuremath{\lambda}}_{\mathrm{eff}}\left(T\right)$ in the layered topological superconductor candidate ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. The exponential temperature dependence of ${\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature ${T}_{c}=2.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the gap to ${T}_{c}$ ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.95(19)$. We also reveal that the ratio ${T}_{c}/{\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}$ 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 $\ensuremath{\mu}\mathrm{SR}$ 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 ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.