Intrinsic Superconducting Correlations in Graphene

by Annica Black‑Schaffer (NORDITA)

Friday 2 Oct 2009, 13:15 → 14:15 Europe/Stockholm

122:026

While not widely appreciated, electronic correlations appear to play an important role in graphene. Indeed, Pauling’s resonance valence bond theory for the pp-bonded planar organic molecules, of which graphene is the infinite extension, already established the importance of the nearest neighbor spin-singlet bond (SB) state in such materials. Through the use of a phenomenological Hamiltonian, which includes SB correlations, we show that a superconducting time-reversal symmetry breaking d-wave state is possible at finite doping in graphene. Ab-initio calculations are then used to study the charge transfer between a graphite sheet and sulfur atoms, demonstrating that the d-wave superconducting state should be achievable in graphite-sulfur structures. These results promise to help to shed light on recent reports of traces of superconductivity, with a remarkably high Tc, in both graphite and graphite-sulfur composites. Recent experimental realizations of SNS graphene Josephson junctions have also prompted our investigation of the possibility of enhancing the intrinsic SB superconducting correlations by coupling to an induced superconducting state. We show that for conventional s-wave contacts a significant interface roughness is necessary in order to achieve any coupling, whereas, for d-wave superconducting contacts, the effects of the SB correlations are significantly enhanced even high above Tc.