Suppression of charge-density wave and superconductivity in a lithiated NbSe$_2$ monolayer

Abstract: We present an \textit{ab initio} investigation of the long-range charge density wave (CDW) order and superconducting properties of the pristine and lithiated NbSe$_2$ monolayer. Stable CDW structures are obtained through atomic reconstruction driven by soft-mode distortions and lithiation, respectively, lead to significant electronic modifications that suppress the CDW order. This suppression is attributed to anisotropic atomic distortions, along with a reduction in the electronic density of states at the Fermi level. As a result, the electron--phonon coupling strength is suppressed, particularly in the lithiated structure, due to reduced contributions from low-frequency phonons, primarily associated with in-plane Nb vibrations. Finally, we observe a sizable anisotropy in the superconducting gap on the Fermi surface, with a superconducting transition temperature of approximately 8~K in the distorted, and 4~K in the lithiated, CDW NbSe$_2$ monolayer.