Electron-phonon coupling and phonon dynamics in single-layer NbSe$_2$ on graphene: the role of moir\'e phonons

TL;DR

This study investigates how moiré phonons and substrate interactions influence electron-phonon coupling and phonon dynamics in monolayer NbSe₂ on graphene, revealing their impact on superconductivity and electronic properties.

Contribution

It provides the first detailed comparison of electron-phonon interactions and phonon dispersion in bulk and monolayer NbSe₂ on graphene, highlighting the role of moiré phonons.

Findings

Reduced electron-phonon coupling in monolayer NbSe₂ on graphene (λ=0.55) compared to bulk (λ=0.76)

Identification of moiré phonons at 1.7 meV contributing to electron-phonon coupling

Superconducting T_C of 1.56 K consistent with coupling strength and transport data

Abstract

The interplay between substrate interactions and electron-phonon coupling in two-dimensional (2D) materials presents a significant challenge in understanding and controlling their electronic properties. Here, we present a comparative study of the structural characteristics, phonon dynamics, and electron-phonon interactions in bulk and monolayer NbSe$_2$ on epitaxial bilayer graphene (BLG) using helium atom scattering (HAS). High-resolution helium diffraction reveals a (9x9)0$^{\circ}$ superstructure within the NbSe$_2$ monolayer, commensurate with the BLG lattice, while out-of-plane HAS diffraction spectra indicate a low-corrugated (3$\sqrt{3}$x3$\sqrt{3}$)30$^{\circ}$ substructure. By monitoring the thermal attenuation of the specular peak across a temperature range of 100 K to 300 K, we determined the electron-phonon coupling constant $\lambda_{HAS}$ as 0.76 for bulk 2H-NbSe$_2$. In contrast, the NbSe$_2$ monolayer on graphene exhibits a reduced $\lambda_{HAS}$ of 0.55, corresponding to a superconducting critical temperature (T$_C$) of 1.56 K according to the MacMillan formula, consistent with transport measurement findings. Inelastic HAS data provide, besides a set of dispersion curves of acoustic and lower optical phonons, a soft, dispersionless branch of phonons at 1.7 meV, attributed to the interface localized defects distributed with the superstructure period, and thus termed moir\'e phonons. Our data show that moir\'e phonons contribute significantly to the electron-phonon coupling in monolayer NbSe$_2$. These results highlight the crucial role of the BLG on the electron-phonon coupling in monolayer NbSe$_2$, attributed to enhanced charge transfer effects, providing valuable insights into substrate-dependent electronic interactions in 2D superconductors.