Effect of nearest neighbor spin-singlet correlations in conventional graphene SNS Josephson junctions

Abstract

Using the self-consistent tight-binding Bogoliubov–de Gennes formalism we have studied the effect of nearest neighbor spin-singlet bond (SB) correlations on Josephson coupling and proximity effect in graphene SNS Josephson junctions with conventional s-wave superconducting contacts. For strong enough coupling the SB correlations give rise to a superconducting state with either an extended s-, dx2−y2-, or dxy-wave symmetry, or different combinations of the d-waves with the dx2−y2+idxy-state favored in the bulk. Despite the s-wave superconducting state in the contacts, the SB pairing state inside the junction has d-wave symmetry and clean, sharp interface junctions resemble a “bulk-meets-bulk” situation with very little interaction between the two different superconducting states. In fact, due to a finite-size suppression of the superconducting state, a stronger SB coupling constant than in the bulk is needed in order to achieve SB pairing in a junction. For both short clean zigzag and armchair junctions the d-wave state that has a zero Josephson coupling to the s-wave state is chosen and therefore the Josephson current decreases when a SB pairing state develops in these junctions. In more realistic junctions, with smoother doping profiles and atomic scale disorder at the interfaces, it is possible to achieve some coupling between the contact s-wave state and the SB d-wave states. In addition, by breaking the appropriate lattice symmetry at the interface in order to induce the other d-wave state, a nonzero Josephson coupling can be achieved which leads to a substantial increase in the Josephson current. We also report on the LDOS of the junctions and on a lack of zero energy states at interfaces despite the unconventional order parameters, which we attribute to the near degeneracy of the two d-wave solutions and their mixing at a general interface.

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