Investigation of the two-gap superconductivity in a few-layer NbSe2 -graphene heterojunction

Abstract

We investigated the superconductivity in a few-layer ${\mathrm{NbSe}}_{2}$-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in ${\mathrm{NbSe}}_{2}$ were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in ${\mathrm{NbSe}}_{2}$ followed the expected temperature behavior in the limit of weak interband scattering, with a gap to ${T}_{c}$ ratio suggesting a weak to moderately strong coupling in few-layer systems.