Electron-Phonon Coupling and Phonon Dynamics in Single-Layer NbSe₂ on Graphene: The Role of Moiré Phonons

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 NbSe₂ on epitaxial bilayer graphene (BLG) using helium atom scattering (HAS). High-resolution helium diffraction reveals a (9 × 9)0° superstructure within the NbSe₂ monolayer, commensurate with the BLG lattice, while out-of-plane HAS diffraction spectra indicate a low-corrugated (3√3 × 3√3)30° substructure. By monitoring the thermal attenuation of the specular peak across a temperature range of 100 to 300 K, we determined the electron-phonon coupling constant (λ_HAS) as 0.76 for bulk 2H-NbSe₂. In contrast, the NbSe₂ monolayer on graphene exhibits a reduced λ_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, thus termed Moiré phonons. Our data show that Moiré phonons contribute significantly to the electron-phonon coupling in monolayer NbSe₂. These results highlight the crucial role of the BLG in the electron-phonon coupling in monolayer NbSe₂, attributed to enhanced charge transfer effects, providing valuable insights into substrate-dependent electronic interactions in 2D superconductors.