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

TL;DR

This study demonstrates that Nb_0.25Bi_2Se_3 is a topological superconductor with a fully gapped, time-reversal invariant state, suggesting unconventional pairing mechanisms without breaking time-reversal symmetry.

Contribution

The paper provides experimental evidence of a fully gapped, time-reversal invariant topological superconducting state in Nb_0.25Bi_2Se_3 using muon spin rotation techniques.

Findings

Fully gapped superconducting state confirmed by muSR

Time-reversal symmetry preserved in the superconducting state

Unconventional pairing suggested by T_c/lambda^{-2} ratio

Abstract

Recently, the niobium (Nb)-doped topological insulator Bi_2Se_3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi_2Se_3 layers, has been shown to exhibit both superconductivity with T_c = 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature-dependent of effective magnetic penetration depth Lambda_eff(T) in the layered topological superconductor candidate Nb_0.25Bi_2Se_3. The exponential temperature dependence of lambda_(eff)^(-2)(T) at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature T_c = 2.9 K and the gap to T_c ratio 2Delta/k_BT_c = 3.95(19). We also revealed that the ratio T_c/lambda_(eff)^(-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 muSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically non-trivial electronic structure of Nb_0.25Bi_2Se_3.