Origin of superconductivity and latent charge density wave in NbS$_2$

TL;DR

This study uses advanced ab initio calculations to reveal that superconductivity in 2H-NbS2 arises from strong electron-phonon interactions at Fermi surface hot spots, with evidence for two-gap behavior linked to specific Fermi pockets.

Contribution

It provides a detailed microscopic mechanism for superconductivity in NbS2, highlighting the role of anharmonic phonons and Fermi surface hot spots, and confirms two-gap superconductivity through theoretical and spectroscopic evidence.

Findings

Superconductivity is linked to Fermi surface hot spots with strong electron-phonon coupling.

Low-energy anharmonic phonons dominate the electron-lattice interaction.

Evidence for two-gap superconductivity associated with different Fermi surface pockets.

Abstract

We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide definitive evidence for two-gap superconductivity in 2$H$-NbS$_2$, and show that the low- and high-energy peaks observed in tunneling spectra correspond to the $\Gamma$- and $K$-centered Fermi surface pockets, respectively. The present findings call for further efforts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides.