Description
While not yet widely appreciated, electronic correlations
appear to play an important role in graphene. In fact,
already Pauling’s resonance valence bond theory established
that nearest-neighbor spin-singlet bond (SB) correlations
are important in pp-bonded planar organic molecules of which
graphene is the infinite extension. 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. DFT calculations are then used to study the
charge transfer between graphene and sulfur, demonstrating
that this d-wave superconducting state should be achievable
in graphite-sulfur structures. We also show that in a d-wave
contact SNS graphene Josephson junction the effects of the
SB correlations are large even high above Tc. We therefore
propose that these junctions will provide a promising
experimental system for measuring the effective strength of
the intrinsic SB correlations.
In addition, we study the magnetic properties of the same
phenomenological Hamiltonian in undoped graphene and we show
that the SB correlations significantly enhance the RKKY
coupling between two impurity magnetic moments. When
matching our results to recent DFT calculations we not only
establish that electronic correlations are essential to
properly account for the behavior of the RKKY coupling but
we also extract a surprisingly large value of the SB
coupling constant, indicating that undoped graphene is
possibly very close to an antiferromagnetic instability.
Primary author
Dr
Annica Black-Schaffer
(Nordita)
