Paola has worked
for more than 10 years on the experimental observation of superconducting
instabilities in carbon nanotubes. She presents here some overview of the field
and its challenges. The observation of intrinsic Superconducting instabilities
in sp2 -hybridized carbon nanophases is a long-standing issue. Hints of
superconducting transitions have been observed so far in many carbon phases but
reproducibility is difficult to obtain and no clear picture of the mechanism
responsible for this transition has emerged so far.
One of the most
promising mechanisms to induce superconductivity is to tune the chemical
potential of the nanostructure close to a Van Hove singularity, where the electron-phonon
coupling is showing some singularity.
In monolayer
graphene, this requires to shift the Fermi Level up to 2eV, a position
impossible so far to obtain experimentally as it requires to electrostatically
dope graphene above 10^15 electrons/cm2. However in twisted graphene bilayers
or in carbon nanotubes, such Van Hove singularities occurs at much lower energy
and are within reach of gating.
Paola then
explains Carbon nanotubes (CNTs) are best candidates to observe the effect of
Van Hove singularities and to tune them. There are atomically thin materials
thus Paola emphasize that there properties varies a lot when one varies their
size by one unit cell. For example in CNTs, the electronic properties are
changing from a semiconductor to a metal with just one more carbon atom along
its perimeter. bandgap is inversely proportional to the diameter of the nanotube and put the first Van Hove
singularity to a fraction of eV.
Paola is
presenting a new set of experiments in which she showed that it is indeed
possible to push a CNT transistor at a Van Hove singularity. This singularity
is detected by the measurement of the capacitance. Indeed the quantum component
of the total capacitance shows a rapid change at the Van Hove singularity. To
confirm that, she has measured the quantum capacitance of the CNT at low
temperature by observing the Fabry-Perot effect. At the singularity the CNT
show some anomaly in the periodic oscillation indicating that a rapid change of
the electronic compressibility is occurring. At low temperature, at this precise gate voltage, she also
observed a zero Bias anomaly associated by a drop of a factor 2 of resistance.
This is associated to a critical temperature of about 30K. A second device probing another portion
of the same nanotube with different length is also showing a similar effect but
with different parameters. Further
measurements will assess for the observation of these instabilities and will
probe deeper the properties of this exotic effect.
Blogged by
Vincent Bouchiat
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