Superconducting proximity effect in a NbSe2/graphene van der Waals junction

Abstract

We investigate the transport properties of the van der Waals (vdW) junction between a layered superconductor $\mathrm{Nb}{\mathrm{Se}}_{2}$ and a graphene. A superconductor/graphene (S/G) vdW junction is fabricated by dry transferring a freshly exfoliated $\mathrm{Nb}{\mathrm{Se}}_{2}$ flake onto a graphene surface. This vdW junction provides a transparent superconductor/graphene contact as well as metal-induced doping in the graphene layer underneath, thereby facilitating vdW coupling-induced superconductivity in the graphene under the $\mathrm{Nb}{\mathrm{Se}}_{2}$ layer. The $\mathrm{Nb}{\mathrm{Se}}_{2}/\text{superconducting}$ graphene (Sc-graphene)/graphene structure causes the differential resistance of the vdW junction to exhibit zero-bias dip and multiple peaks at larger bias. All of these features are explained by the coexistence of two different S/G interfaces in the device; these are lateral Sc-graphene/graphene and vertical $\mathrm{Nb}{\mathrm{Se}}_{2}/\mathrm{Sc}$-graphene. A proximity-induced superconducting gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{i}}$ in the Sc-graphene is detected by Andreev reflection at the lateral Sc-graphene/graphene junction and the determined ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{i}}=0.05\ensuremath{-}0.06\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$.