Physical properties of orthorhombic NbS: An investigation using first-principles calculations

We scrutinized the structural, electronic, elastic, mechanical, phononic, and electron-phonon coupling properties of orthorhombic NbS by executing first-principles calculations based on the density functional theory with its generalized gradient approximation. The Nb $4d$ electron states mainly contribute to the energy bands near the Fermi level. This result suggests that the electrons in the Nb $4d$ states mostly dominate the charge conduction and superconducting state properties in the studied compound. An analysis of elastic constants reveals the mechanical stability of NbS in its orthorhombic MnP-type crystal structure. In contrast, an analysis of elastic moduli indicates that this intermetallic compound has intrinsic ductility. Our phonon calculations prove the dynamical stability of NbS in its orthorhombic MnP-type crystal structure. A comparison of the phonon density of states and Eliashberg electron-phonon interaction spectral function underscores the superconducting trait of orthorhombic NbS is attributed to the vibrational modes of Nb atoms, which strongly mediate the formation of Cooper pairs due to the dominance of Nb $4d$ electrons at the Fermi level. Furthermore, our electron-phonon interaction calculations reveal that the orthorhombic NbS is a medium-coupling electron-phonon-mediated superconductor with an electron-phonon coupling parameter of 0.83 and has a superconducting transition temperature of 6.9 K, which is comparable with its recently measured value of 6.0 K.