Phonon-mediated superconductivity in orthorhombic XS (X = Nb, Ta or W)

Abstract The unique three-dimensional orthorhombic NbS (o-NbS) compound synthesized in 1969 has recently been experimentally confirmed to be a superconductor [ Phys. Rev. B 108 174517 (2023)]. However, there is currently no theoretical research on its superconducting mechanism. In this work, we investigate the superconducting properties of o-NbS from first-principles calculations. Based on the Eliashberg equation, it is found that the superconductivity mainly originates from the coupling between the electrons of Nb-4d orbitals and the vibrations of Nb atoms in the low-frequency region and those of S atoms in the high-frequency region. A superconducting transition temperature ( T c ) of 10.7 K is obtained, which is close to the experimental value and higher than most transition metal chalcogenides (TMCs). The calculated thermodynamic properties in the superconducting state, such as specific heat, energy gap, isotope coefficient, etc., also indicate that o-NbS is a conventional phonon-mediated superconductor. These results are consistent with recent experimental reports and provide a good understanding of the superconducting mechanism of o-NbS. Furthermore, the TMCs of o-TaS and o-WS are also investigated; these belong to the same and neighboring groups of Nb, and we find that o-TaS and o-WS are also phonon-mediated superconductors with T c of 8.9 K and 7.2 K, respectively.