Crystal
structure of Ca-intercalated bilayer graphene fabricated
on SiC
substrate. Insertion of Ca atoms between two graphene
layers causes
the superconductivity.
(February 16, 2016) Graphene
is a single-atomic carbon sheet with a hexagonal honeycomb network (Fig. 1).
Electrons in graphene take a special electronic state called Dirac-cone where
they behave as if they have no mass. This allows them to flow at very high
speed, giving graphene a very high level of electrical conductivity.
This is significant because electrons with no mass flowing
with no resistance in graphene could lead to the realization of an ultimately
high-speed nano electronic device.
The collaborative team of Tohoku University and the University
of Tokyo has developed a method to grow high-quality graphene on a silicon
carbide (SiC) crystal by controlling the number of graphene sheets. The team
fabricated bilayer graphene with this method and then inserted calcium (Ca)
atoms between the two graphene layers like a sandwich (Fig. 1).
They measured the electrical conductivity with the micro
four-point probe method and found that the electrical resistivity rapidly drops
at around 4 K (-269 °C), indicative of an emergence of superconductivity (Fig.
2).
Figure 2: Temperature
dependence of electrical resistivity of
Ca-intercalated
bilayer graphene, measured by the micro four-point-probe
method (inset).
The resistivity shows a rapid decrease at around 4 K and
reaches
"zero" at 2 K, showing the emergence of superconductivity.
The team also found that neither genuine bilayer graphene
nor lithium-intercalated bilayer graphene shows superconductivity, indicating
that the superconductivity is driven by the electron transfer from Ca atoms to
graphene sheets.
The success in fabricating superconducting graphene is
expected to greatly impact both the basic and applied researches of graphene.
It is currently not clear what phenomenon takes place when
the Dirac electrons with no mass become superconductive with no resistance. But
based on the latest study results, further experimental and theoretical
investigations would help to unravel the properties of superconducting
graphene.