Superconducting and normal state properties of the noncentrosymmetric superconductor NbOs2 investigated by muon spin relaxation and rotation

Noncentrosymmetric superconductors with $\ensuremath{\alpha}$-manganese structure has attracted much attention recently, after the discovery of time-reversal symmetry breaking in all the members of the ${\mathrm{Re}}_{6}X$ ($X=\text{Ti}$, Hf, Zr) family. Similar to ${\mathrm{Re}}_{6}X$, ${\mathrm{NbOs}}_{2}$ also adopts $\ensuremath{\alpha}\text{\ensuremath{-}}\text{Mn}$ structure and is found to be superconducting with critical temperature ${T}_{c}\ensuremath{\simeq}2.7$ K. The results of the resistivity, magnetization, specific heat, and muon spin relaxation/rotation measurements show that ${\mathrm{NbOs}}_{2}$ is a weakly coupled type-II superconductor. Interestingly, the zero-field muon experiments indicate that the time-reversal symmetry is preserved in the superconducting state. The low-temperature transverse-field muon measurements and the specific-heat data evidence a conventional isotropic fully gapped superconductivity. However, the calculated electronic properties in this material show that the ${\mathrm{NbOs}}_{2}$ is positioned close to the band of unconventionality of the Uemura plot, indicating that ${\mathrm{NbOs}}_{2}$ potentially borders an unconventional superconducting ground state.