Abstract
We investigate the electric properties through graphene-Nb superconductor junction and aim to study Andreev reflection (AR) as a function of Fermi-level EF of graphene. The Nb thin film is deposited on quartz with superconducting temperature Tc close to 9 K. The graphene is prepared by chemical vapor deposition (CVD) and transfer on top of Nb-pattern, in which two series coupled junctions are defined. In particular, we carefully treat Nb surface so to ensure clean interface is achieved. To control EF of graphene, we employ ion-gel on graphene as a gate electrode. We first characterize the resistance of graphene with the gate voltage Vg, determine EF respected to the charge neutral point (CNP) and then cool down the sample below Tc,. We find as EF is tuned away CNP, the differential conductivity dI/dV v.s. V shows a suppressed dip structures, which width is comparable with the expected superconducting gap of Nb. Distinctly, as EF is in the vicinity of CNP, the downward feature of dI/dV becomes more pronounced. Data analysis suggest the retro-AR is enhanced near CNP due to smaller the critical angle of Andreev reflection. Our results provide the first comprehensive studies of electric properties of graphene-supercondutor interface and open a new route to further explore the tunneling processes of AR for Dirac fermions.