Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe₂ Films

Abstract

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 2 O 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 ±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.