Superconductivity enhanced conductance fluctuations in few layer graphene nanoribbons

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We investigate the mesoscopic disorder induced rms conductance variance δG in a few layer graphene nanoribbon (FGNR) contacted by two superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we observe pronounced conductance fluctuations superimposed on a linear background of the two terminal conductance G. The linear gate-voltage induced response can be modeled by a set of inter-layer and intra-layer capacitances. δG depends on temperature T and source-drain voltage Vsd. δG increases with decreasing T and |Vsd|. When lowering |Vsd|, a pronounced cross-over at a voltage corresponding to the superconducting energy gap Δ is observed. For |Vsd| Δ the fluctuations are markedly enhanced. Expressed in the conductance variance GGS of one graphene-superconducutor (G-S) interface, values of 0.58 e²/h are obtained at the base temperature of 230 mK. The conductance variance in the sub-gap region are larger by up to a factor of 1.4-1.8 compared to the normal state. The observed strong enhancement is due to phase coherent charge transfer caused by Andreev reflection at the nanoribbon-superconductor interface.