Abstract
We implement a tunable vertical graphene-superconductor tunnel junction and aim to investigate the tunneling behaviors between Dirac fermions and Cooper pairs. Our device consists with two serial coupled Niobium (Nb)-Graphene junctions, which are coated with an ionic gel as a top-gate electrode for controlling the Fermi-level (EF) of grapheme at room temperature. When EF is tuned away from the charge-neutral point (CNP or called the Dirac point) of grapheme and the temperature is lower to the critical temperature of Nb (~ 9 K), we observe a clear dip structure of differential conductance in the vicinity of zero-bias source-drain voltage (VSD), and peak features as VSD approaches the superconducting gap, which indicate clear behaviors of a superconducting Nb-graphene junction. The general features aforementioned evolve with EF ; intriguingly the differential conductance becomes fluctuating as EF is tuned closed CNP. Our data analysis suggests the observed tunneling behaviors are consistent with the theoretical predictions based on the approaches of Bogoliubov-de-Genn equation when EF is away from CNP. Near CNP, the tunneling behaviors are dominated by the combinations of the effects of interfacial disorders and specular Andreev reflection.