Scanning tunneling microscopy and spectroscopy study of graphene/NbSe₂ heterostructures

We investigate vertical van der Waals heterostructures consisting of monolayer graphene and bulk NbSe₂. Due to interactions between the layers, coherent states in NbSe₂, including charge density wave (CDW) and superconductivity, can be inherited by the graphene. The graphene/NbSe₂ heterostructures were characterized with liquid helium temperature scanning tunneling microscopy. The graphene lattice, NbSe₂ lattice and a moiré pattern due to their lattice mismatch are observed by scanning tunneling microscopy measurements. A CDW is also observed in the graphene, which is aligned with the NbSe₂ lattice and 3 times its length, indicating it originates from the NbSe₂. From the scanning tunneling spectroscopy measurements, a 1.1 meV superconducting gap is observed in the graphene. Vortices are also observed in a perpendicular magnetic field of 0.3 T. The superconductivity in the magnetic field is further confirmed by electrical transport measurements, which show a sharp drop in resistance at 6.2 K.