Superconductivity in orthorhombic NbS

Compared to the intensively studied niobium dichalcogenides, the physical properties of niobium monosulfide (NbS) have been less explored. Here we report the discovery of bulk superconductivity in orthorhombic NbS ($o$-NbS). First-principles results of $o$-NbS are also revealed. $o$-NbS crystallizes in a MnP-type structure ($Pnma$) with lattice parameters $a=\text{6.4450(1)}$ \AA{}, $b=\text{3.3 1770(4)}$ \AA{}, and $c=\text{5.8629(1)}$ \AA{}. It exhibits type-II superconductivity with a critical temperature (${T}_{\mathrm{c}}$) of 6.0 K. Bulk superconductivity is confirmed by the magnetization and heat capacity measurements. The upper and lower critical fields are 7.4 T and 13.0 mT, respectively. Specific heat data suggest that $o$-NbS is a fully gapped $s$-wave superconductor, with ${\mathrm{\ensuremath{\Delta}}}₀/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=1.75$ and $\mathrm{\ensuremath{\Delta}}{C}_{\mathrm{e}}/\ensuremath{\gamma}{T}_{\mathrm{c}}=1.45$, fitting perfectly within the BCS theory. First-principles calculations indicate that the $\mathrm{Nb}\text{\ensuremath{-}}4d$ states dominate the Fermi level (${E}_{\mathrm{F}}$), and the density of states (DOS) on ${E}_{\mathrm{F}}$ is sensitive to niobium vacancies. Different from the quasi-two-dimensional $2H\text{\ensuremath{-}}{\mathrm{NbS}}₂$ and quasi-one-dimensional ${\mathrm{Nb}}₃{\mathrm{S}}₄$ superconductors, our discovery of superconductivity in three-dimensional $o$-NbS provides a different motif for both experimental and theoretical studies.