# Electron-phonon coupling and the Coexistence of Superconductivity and Charge-Density Wave in Monolayer NbSe2

## TL;DR

This paper investigates how electron-phonon interactions influence the coexistence of superconductivity and charge-density waves in monolayer NbSe2, revealing the mechanisms behind their interplay and effects on electronic properties.

## Contribution

It provides a detailed first-principles analysis of electron-phonon coupling in monolayer NbSe2, elucidating the mechanisms of coexistence and competition between superconductivity and CDW phases.

## Key findings

- Intra-pocket scattering relates to superconductivity with constant gaps.
- Inter-pocket scattering causes CDW and bandgap formation.
- Fermi surface gapping explains the transition to two-gap superconductivity.

## Abstract

Monolayer 2H-NbSe2 has recently been shown to be a 2-dimensional superconductor, with a coexisting charge-density wave (CDW). As both phenomena are intimately related to electron-lattice interaction, a natural question is how superconductivity and CDW are interrelated through electron-phonon coupling (EPC), which is important to the understanding of 2-dimensional superconductivity. This work investigates the superconductivity of monolayer NbSe2 in CDW phase using the anisotropic Migdal-Eliashberg formalism based on first principles calculations. The mechanism of the competition between and coexistence of the superconductivity and CDW is studied in detail by analyzing EPC. It is found that the intra-pocket scattering is related to superconductivity, leading to almost constant value of superconducting gaps on parts of the Fermi surface. The inter-pocket scattering is found to be responsible for CDW, leading to partial or full bandgap on the remaining Fermi surface. Recent experiment indicates that there is transitioning from regular superconductivity in thin-film NbSe2 to two-gap superconductivity in the bulk, which is shown here to have its origin in the extent of Fermi surface gapping of K and K' pockets induced by CDW. Overall blue shifts of the phonons and sharp decrease of Eliashberg spectrum are found when the CDW forms.