Probing the Two-gap Superconductivity in a Few-layer NbSe₂-graphene Heterojunction

2H-NbSe₂, a layered transition metal dichalcogenide, offers an ideal platform for the study of superconductivity from a bulk system to two-dimensional limit. In our work, differential conductance spectroscopy was exploited to probe the superconductivity in a few-layer NbSe₂-graphene heterojunction. Owing to the gate-tunable Fermi level of few-layer graphene, the sensitively gate-dependent differential conductance allowed us to investigate the superconducting gap structure of NbSe₂ with continuously tuned junction transparency between tunneling regime and Andreev reflection limit. Features of two-gap superconductivity in NbSe₂ were observed with a characteristic differential conductance of a central dip and two sets of coherence peaks when the Fermi level was finely tuned to the charge neutrality point of the junction. Through Blonder-Tinkham-Klapwijk fits, two gaps along with their temperature dependence were extracted. The gap to T_c ratio resulted from the fits and the temperature behavior of the two gaps were further analyzed and discussed, pointing to a weak to moderately strong coupling scenario for the few-layer NbSe₂.