Wednesday 29 July 2015 – Vincent Bouchiat - Field-effect controlled superconductivity in metal-decorated graphene

The seminar by Vincent Bouchiat on “Field effect controlled superconductivity in metal decorated graphene” consisted a systematic study of proximity-induced superconductivity in graphene created with either randomly distributed or lithographically patterned arrays of tin nanoparticles/discs. The underlying idea of the work is very interesting because it exploits the field effect properties of graphene, as well as the tunability of disorder, to explore several contemporary questions and issues in purely two dimensional superconductivity, ranging from the nature quantum phase transition from the superconducting to the metallic (or the insulating) state to granular superconductivity and emergent intermediate metallic state.

The talk was laid out nicely with appropriate chronological and conceptual sequence. Dr. Bouchiat first described the work on graphene decorated with dense randomly distributed Sn adsorbates. The 2D nature of proximity-induced superconductivity in graphene seem to favor phase fluctuations and a gate tunable Berezinskii-Kosterlitz-Thouless (BKT)-type scaling of the temperature-dependence of resistance across the transition. It was then shown that similar experimental platform can be used to achieve a superconductor-insulator transition too, by using deliberately damaged CVD graphene (via chemical means) as the host material. It was interesting to see that the transition occurred at the universal magnitude of h/4e², expected from the self-dual Bosonic scenario, although the universality of this observation was not clear to me. Another interesting aspect was the exponent obtained from the finite size scaling, which did not correspond to the classical percolation picture. The speaker speculated if this could correspond to quantum percolation processes.

The final experimental aspect was superconductivity induced in graphene by a dilute but regular array of the Sn dots. The most interesting observation here is the anomalous reduction of the sample's critical temperature T_c with respect to the calculated T_BKT on increasing graphene resistance, leading to a sudden collapse of superconductivity when approaching the charge neutrality point of the graphene layer. Dr. Bouchiat believes this deviation is due to the breakdown of the superconducting phase stiffness due to the emergence of quantum phase fluctuations, rather than thermal phase fluctuations which is addressed in conventional BKT-type framework.

In summary, I wish to note that the use of graphene as the experimental platform seems to be motivated mainly by the 2D nature of graphene rather than its unique band structure or Dirac Fermionic quasiparticle excitations. For example, Cooper pairs in graphene’s hexagonal lattice are expected to give rise to nontrivial Andreev reflection, which were not encountered in these experiments. The experiments presented in the seminar not only demonstrate very similar features observed and debated in other classes of low-dimensional superconductors such as the oxide interfaces or ultra-thin NbN films, but also emphasize the applicability of graphene as a versatile system with which many of the outstanding questions can be investigated with great control. 

Blogged by Arindam Ghosh