Temperature dependence of proximity-induced supercurrent in single and multi-layer graphene*

Graphene is an attracting material for the superconducting proximity effect. In single layer graphene (SLG), the peculiar band structure leads to the relativistic Josephson effect, while in multilayer graphene (MLG), the layered structure with large modulation of carrier density from negative to positive values provides a novel situation of conventional proximity effect. Here we present experimental study on superconducting proximity effect in SLG and MLG. For SLG with junction length of 220 nm, we observed gate-voltage dependent critical supercurrent $I_c$, and its temperature dependences for all gate voltages were well explained by a conventional theory for short and dirty junctions (KO1 theory). On the other hand, in MLG junctions, $I_c(T) \propto \exp (-(T/T_0)^2)$, where $T_0$ is a sample- and gate- dependent constant. This behavior can be explained by a successive transition model, in which a graphene layer with larger carrier density has a higher temperature for the onset of supercurrent.