Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe_2 Films

Understanding the interplay between superconductivity and charge-density wave (CDW) in NbSe_2 is vital for both fundamental physics and future device applications. Here, combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy and Raman spectroscopy, we study the CDW phase in the monolayer NbSe_2 films grown on various substrates of bilayer graphene (BLG), SrTiO_3(111), and Al_2O_3(0001). It is found that the two stable CDW states of monolayer NbSe_2 can coexist on NbSe_2/BLG surface at liquid-nitrogen temperature. For the NbSe_2/SrTiO_3(111) sample, the unidirectional CDW regions own the kinks at \pm 41 meV and a wider gap at 4.2 K. It is revealed that the charge transfer from the substrates to the grown films will influence the configurations of the Fermi surface, and induce a 130 meV lift-up of the Fermi level with a shrink of the Fermi pockets in NbSe_2/SrTiO_3(111) compared with the NbSe_2/BLG. Combining the temperature-dependent Raman experiments, we suggest that the electron-phonon coupling in monolayer NbSe_2 dominates its CDW phase transition.