Tunneling Measurements in Single- and Multi-layer Graphene

We have fabricated novel single- and multi-layer graphene devices with both normal metal ohmic contacts and superconducting tunnel probes. The superconducting gap of the tunnel probes in the graphene devices is well formed at 250 mK, showing that the probes are good for tunneling spectroscopy studies of graphene electronic structure. We have observed oscillations as a function of bias and gate voltages, possibly due to electron phase interference among the layers and/or tunnel probe interfaces. Those oscillations die out in the presence of magnetic fields. Unexpectedly, two distinct and symmetric peaks, which have weak dependence on gate voltages, exist in the superconducting gap.