New findings for “magic angle” trilayer graphene might help inform the design of more powerful MRI machines or robust quantum computers.
By Jennifer Chu, MIT News Office
July
21, 2021 -- MIT physicists have observed signs of a rare type of
superconductivity in a material called magic-angle twisted trilayer graphene.
In a study appearing today in Nature, the researchers report that
the material exhibits superconductivity at surprisingly high magnetic fields of
up to 10 Tesla, which is three times higher than what the material is predicted
to endure if it were a conventional superconductor.
The results strongly imply that
magic-angle trilayer graphene, which was initially discovered by the same
group, is a very rare type of superconductor, known as a “spin-triplet,” that
is impervious to high magnetic fields. Such exotic superconductors could vastly
improve technologies such as magnetic resonance imaging, which uses
superconducting wires under a magnetic field to resonate with and image
biological tissue. MRI machines are currently limited to magnet fields of 1 to
3 Tesla. If they could be built with spin-triplet superconductors, MRI could
operate under higher magnetic fields to produce sharper, deeper images of the
human body.
The new evidence of spin-triplet
superconductivity in trilayer graphene could also help scientists design
stronger superconductors for practical quantum computing.
“The value of this experiment is what it
teaches us about fundamental superconductivity, about how materials can behave,
so that with those lessons learned, we can try to design principles for other
materials which would be easier to manufacture, that could perhaps give you
better superconductivity,” says Pablo Jarillo-Herrero, the Cecil and Ida Green
Professor of Physics at MIT.
His co-authors on the paper include
postdoc Yuan Cao and graduate student Jeong Min Park at MIT, and Kenji Watanabe
and Takashi Taniguchi of the National Institute for Materials Science in Japan.
Strange shift
Superconducting materials are defined by
their super-efficient ability to conduct electricity without losing energy.
When exposed to an electric current, electrons in a superconductor couple up in
“Cooper pairs” that then travel through the material without resistance, like
passengers on an express train.
In a vast majority of superconductors,
these passenger pairs have opposite spins, with one electron spinning up, and
the other down — a configuration known as a “spin-singlet.” These pairs happily
speed through a superconductor, except under high magnetic fields, which can
shift the energy of each electron in opposite directions, pulling the pair
apart. In this way, and through mechanisms, high magnetic fields can derail
superconductivity in conventional spin-singlet superconductors.
“That’s the ultimate reason why in a
large-enough magnetic field, superconductivity disappears,” Park says.
But there exists a handful of exotic
superconductors that are impervious to magnetic fields, up to very large
strengths. These materials superconduct through pairs of electrons with the
same spin — a property known as “spin-triplet.” When exposed to high magnetic
fields, the energy of both electrons in a Cooper pair shift in the same
direction, in a way that they are not pulled apart but continue superconducting
unperturbed, regardless of the magnetic field strength.
Jarillo-Herrero’s group was curious
whether magic-angle trilayer graphene might harbor signs of this more unusual
spin-triplet superconductivity. The team has produced pioneering work in the
study of graphene moiré structures — layers of atom-thin carbon lattices that,
when stacked at specific angles, can give rise to surprising electronic
behaviors.
The researchers initially reported such
curious properties in two angled sheets of graphene, which they dubbed magic-angle
bilayer graphene. They soon followed up with tests of trilayer
graphene, a sandwich configuration of three graphene sheets that turned out
to be even stronger than its bilayer counterpart, retaining superconductivity
at higher temperatures. When the researchers applied a modest magnetic field,
they noticed that trilayer graphene was able to superconduct at field strengths
that would destroy superconductivity in bilayer graphene.
“We thought, this is something very
strange,” Jarillo-Herrero says.
A super comeback
In their new study, the physicists
tested trilayer graphene’s superconductivity under increasingly higher magnetic
fields. They fabricated the material by peeling away atom-thin layers of carbon
from a block of graphite, stacking three layers together, and rotating the
middle one by 1.56 degrees with respect to the outer layers. They attached an
electrode to either end of the material to run a current through and measure
any energy lost in the process. Then they turned on a large magnet in the lab,
with a field which they oriented parallel to the material.
As they increased the magnetic field
around trilayer graphene, they observed that superconductivity held strong up
to a point before disappearing, but then curiously reappeared at higher field
strengths — a comeback that is highly unusual and not known to occur in
conventional spin-singlet superconductors.
“In spin-singlet superconductors, if you
kill superconductivity, it never comes back — it’s gone for good,” Cao says.
“Here, it reappeared again. So this definitely says this material is not
spin-singlet.”
They also observed that after
“re-entry,” superconductivity persisted up to 10 Tesla, the maximum field
strength that the lab’s magnet could produce. This is about three times higher
than what the superconductor should withstand if it were a conventional
spin-singlet, according to Pauli’s limit, a theory that predicts the maximum
magnetic field at which a material can retain superconductivity.
Trilayer graphene’s reappearance of
superconductivity, paired with its persistence at higher magnetic fields than
predicted, rules out the possibility that the material is a run-of-the-mill
superconductor. Instead, it is likely a very rare type, possibly a
spin-triplet, hosting Cooper pairs that speed through the material, impervious
to high magnetic fields. The team plans to drill down on the material to
confirm its exact spin state, which could help to inform the design of more
powerful MRI machines, and also more robust quantum computers.
“Regular quantum computing is super
fragile,” Jarillo-Herrero says. “You look at it and, poof, it disappears. About
20 years ago, theorists proposed a type of topological superconductivity that,
if realized in any material, could [enable] a quantum computer where states
responsible for computation are very robust. That would give infinite more
power to do computing. The key ingredient to realize that would be spin-triplet
superconductors, of a certain type. We have no idea if our type is of that
type. But even if it’s not, this could make it easier to put trilayer graphene
with other materials to engineer that kind of superconductivity. That could be
a major breakthrough. But it’s still super early.”
This research was supported by the U.S.
Department of Energy, the National Science Foundation, the Gordon and Betty
Moore Foundation, the Fundacion Ramon Areces, and the CIFAR Quantum Materials
Program.
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