Chemistry Reference
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finds that in the regime of physical parameter corresponding to graphene,
the system appears to flow towards a
d
+
id
superconducting state as the
temperature is lowered.
Our results shows that there is an adiabatic continuity of strong cou-
pling superconductivity to intermediate coupling in 2d repulsive Hubbard
model on the honey comb lattice.
What is remarkable is that supercon-
ductivity at low coupling is not exponentially suppressed
. This seems to
be unique to 2 dimensions. We have checked this for 2d square lattice
repulsive Hubbard model as well, using existing numerical results. What
about 3 dimensions? Preliminary analysis shows that the development
of near neighbor singlet correlations is much reduced in 3 dimensions.
That is, for a given ratio of band width to U, the strength of the in-
duced near neighbor singlet correlations seems to get suppressed as we go
from 2 to 3 dimensions, particularly at intermediate or weak couplings.
This is also corraborated by the very low Tc one sees in intercalated
graphite. Intercalated graphite can be viewed as a set of doped graphene
layers. Maximum
T
c
obtained in these systems is around 16 K.
[
52;
53
]
Systems such as CaC
6
has a doping close to the optimal doping of
our theoretical result. Infact, if one looks at the doping vs Tc, by collecting
various intercalated graphite systems, Tc has a dome like structure, very
much like Fig. 8! Why is the
T
c
so low? A closer inspection suggests to
us two possible reasons in CaC
6
: (i) an enhanced 3 dimensionality aris-
ing through the intercalant orbitals and (ii) competition from intercalant
induced charge density wave order.
Superconducting signals with a
T
c
around 60 K and higher have
been reported in the past in pyrolitic graphite containing sulfur.
[
54;
55
]
Sulfur doped graphite, however, gives a hope that there is a possibility
of high temperature superconductivity. Our present theoretical prediction
should encourage experimentalists to study graphite from superconductiv-
ity point of view systematically, along the line pioneered by Kopelevich
and collaborators.
[
55
]
In the past there has been claims (unfortunately
not reproducible) of Josephson like signals in graphite and carbon based
materials;
[
56
]
Again, our result should encourage revival of studies along
these lines.
Simple doping of a freely hanging graphene layer by gate control to the
desired optimal doping of 10 - 20 % is not experimentally feasible at the
present moment. It will be interesting to discover experimental methods
that will allow us to attain these higher doping values. A simple estimate
shows that a large cohesion energy arising from the strong
σ
bond that
