Vincent Bouchiat: Tunable 2D superconductivity in metal-decorated graphene

Bouchiat presented experimental results from recent transport studies on superconductivity in thin metals deposited on top of 2D graphene layers. Gated graphene is used as 2D host material mediating interactions between adsorbed metal islands. These metal islands both efficiently dope the underlying graphene sheet and induce long range superconducting correlations.The fabrication of these systems were done in collaboration with Zettl's group at Berkeley. The dewetting of metals such as Pb, In, Sn when deposited on graphene allows non percolating islands of adsorbates to be formed on graphene with typical sizes of ~ 50 nm with inter-adsorbate distances ~ 80 nm (for Sn). With Sn islands on graphene, a gate-induced superconductor-normal transition was observed. When plotting resistane vs temperature, two drops in R are observed: one at T_c, corresponding to the bulk superconducting transition in the clusters and a second one below T_c, at T_BKT, where vortex unbinding occurs. By tuning the gate voltage and thereby the carrier density in the graphene sheets, it is found that T_c remains constant but T_BKT can be tuned on both the electron and hole branches on either side of the charge neutrality point in graphene.

In the second part of the talk, more disordered graphene layers grown by CVD method is used to study Pb clusters. The interesting results are: (1) Dirac point is found to be an insulating point and (2) superconductivity in found only on one side of the charge neutrality point. A gate-tunable superconductor-insulator transition was observed and the graphene layer was found to act as both Josephson and dissipation channels.

In summary, in clean exfoliated graphene, a gate controlled 2D superconductivity - BKT transition was observed and in disordered CVD grown graphene, a gate-controlled superconductor-insuator transition is observed. Metal decorated graphene appears to be a useful model system to study gate-controlled superconductivity. Some results presented have appeared in Phys. Rev. Lett. 104, 047001 (2010).

Blogged by Sambandamurthy (Buffalo)