NbReSi: A noncentrosymetric superconductor with large upper critical field

We report the discovery of superconductivity in noncentrosymmetric NbReSi, which crystallizes in a hexagonal ZrNiAl-type crystal structure with space group $P\overline{6}2m$ (No. 189). Bulk superconductivity, with ${T}_c=6.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$ was characterized via electrical-resistivity, magnetization, and heat-capacity measurements. The low-temperature electronic specific heat suggests a fully gapped superconducting state in NbReSi, while a large upper critical field of ${\ensuremath{\mu}}₀{H}_{\mathrm{c}2}(0)\ensuremath{\sim}12.6 \mathrm{T}$ is obtained, which is comparable to the weak-coupling Pauli limit. The electronic band-structure calculations show that the density of states at the Fermi level are dominated by Re and Nb $d$ orbitals, with a sizable band splitting induced by the antisymmetric spin-orbit coupling. NbReSi represents another candidate material for revealing the puzzle of time-reversal symmetry breaking observed in some Re-based superconductors and its relation to the lack of inversion symmetry.