Superconductivity and strong anharmonicity in novel Nb-S phases

Abstract: In this work we explore the phase diagram of the binary Nb-S system from ambient pressures up to 250 GPa using $ab$ $initio$ evolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pressure phases of pure sulfur, are metals with low electron-phonon coupling strengths and critical superconducting temperatures below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on lattice dynamics, electron-phonon interactions, and superconductivity for the novel high-pressure phase of $\mathrm{Nb}_2\mathrm{S}$, demonstrating that the inclusion of anharmonicity stabilizes the lattice and enhances the electron-phonon interaction.