Insulator or Superconductor?
Physicists Find Graphene is Both
When rotated at a "magic angle," graphene sheets can form an insulator or a superconductor.
By Jennifer Chu | MIT News Office
March 5, 2018 -- It’s hard to believe that a single material can be described by as many superlatives as graphene can. Since its discovery in 2004, scientists have found that the lacy, honeycomb-like sheet of carbon atoms — essentially the most microscopic shaving of pencil lead you can imagine — is not just the thinnest material known in the world, but also incredibly light and flexible, hundreds of times stronger than steel, and more electrically conductive than copper.
Physicists Find Graphene is Both
When rotated at a "magic angle," graphene sheets can form an insulator or a superconductor.
By Jennifer Chu | MIT News Office
March 5, 2018 -- It’s hard to believe that a single material can be described by as many superlatives as graphene can. Since its discovery in 2004, scientists have found that the lacy, honeycomb-like sheet of carbon atoms — essentially the most microscopic shaving of pencil lead you can imagine — is not just the thinnest material known in the world, but also incredibly light and flexible, hundreds of times stronger than steel, and more electrically conductive than copper.
Now physicists at MIT and Harvard University have found the wonder
material can exhibit even more curious electronic properties. In two papers
published today in Nature, the team reports it can tune graphene to
behave at two electrical extremes: as an insulator, in which electrons are
completely blocked from flowing; and as a superconductor, in which electrical
current can stream through without resistance.
Researchers in the past, including
this team, have been able to synthesize graphene superconductors by placing the
material in contact with other superconducting metals — an arrangement that
allows graphene to inherit some superconducting behaviors. This time around,
the team found a way to make graphene superconduct on its own, demonstrating
that superconductivity can be an intrinsic quality in the purely carbon-based
material.
The physicists accomplished this by
creating a “superlattice” of two graphene sheets stacked together — not
precisely on top of each other, but rotated ever so slightly, at a “magic
angle” of 1.1 degrees. As a result, the overlaying, hexagonal honeycomb pattern
is offset slightly, creating a precise moiré configuration that is predicted to
induce strange, “strongly correlated interactions” between the electrons in the
graphene sheets. In any other stacked configuration, graphene prefers to remain
distinct, interacting very little, electronically or otherwise, with its
neighboring layers.
The team, led by Pablo
Jarillo-Herrero, an associate professor of physics at MIT, found that when
rotated at the magic angle, the two sheets of graphene exhibit nonconducting
behavior, similar to an exotic class of materials known as Mott insulators.
When the researchers then applied voltage, adding small amounts of electrons to
the graphene superlattice, they found that, at a certain level, the electrons
broke out of the initial insulating state and flowed without resistance, as if
through a superconductor.
“We can now use graphene as a new
platform for investigating unconventional superconductivity,” Jarillo-Herrero
says. “One can also imagine making a superconducting transistor out of
graphene, which you can switch on and off, from superconducting to insulating.
That opens many possibilities for quantum devices.”
A 30-year gap
A material’s ability to conduct
electricity is normally represented in terms of energy bands. A single band
represents a range of energies that a material’s electrons can have. There is
an energy gap between bands, and when one band is filled, an electron must
embody extra energy to overcome this gap, in order to occupy the next empty
band.
A material is considered an insulator
if the last occupied energy band is completely filled with electrons.
Electrical conductors such as metals, on the other hand, exhibit partially
filled energy bands, with empty energy states which the electrons can fill to
freely move.
Mott insulators, however, are a
class of materials that appear from their band structure to conduct
electricity, but when measured, they behave as insulators. Specifically, their
energy bands are half-filled, but because of strong electrostatic interactions
between electrons (such as charges of equal sign repelling each other), the
material does not conduct electricity. The half-filled band essentially splits
into two miniature, almost-flat bands, with electrons completely occupying one
band and leaving the other empty, and hence behaving as an insulator.
“This means all the electrons are
blocked, so it’s an insulator because of this strong repulsion between the
electrons, so nothing can flow,” Jarillo-Herrero explains. “Why are Mott
insulators important? It turns out the parent compound of most high-temperature
superconductors is a Mott insulator.”
In other words, scientists have
found ways to manipulate the electronic properties of Mott insulators to turn
them into superconductors, at relatively high temperatures of about 100 Kelvin.
To do this, they chemically “dope” the material with oxygen, the atoms of which
attract electrons out of the Mott insulator, leaving more room for remaining
electrons to flow. When enough oxygen is added, the insulator morphs into a
superconductor. How exactly this transition occurs, Jarillo-Herrero says, has
been a 30-year mystery.
“This is a problem that is 30 years
and counting, unsolved,” Jarillo-Herrero says. “These high-temperature
superconductors have been studied to death, and they have many interesting
behaviors. But we don’t know how to explain them.”
A precise rotation
Jarillo-Herrero and his colleagues
looked for a simpler platform to study such unconventional physics. In studying
the electronic properties in graphene, the team began to play around with
simple stacks of graphene sheets. The researchers created two-sheet
superlattices by first exfoliating a single flake of graphene from graphite,
then carefully picking up half the flake with a glass slide coated with a
sticky polymer and an insulating material of boron nitride.
They then rotated the glass slide
very slightly and picked up the second half of the graphene flake, adhering it
to the first half. In this way, they created a superlattice with an offset
pattern that is distinct from graphene’s original honeycomb lattice.
The team repeated this experiment,
creating several “devices,” or graphene superlattices, with various angles of
rotation, between 0 and 3 degrees. They attached electrodes to each device and
measured an electrical current passing through, then plotted the device’s
resistance, given the amount of the original current that passed through.
“If you are off in your rotation
angle by 0.2 degrees, all the physics is gone,” Jarillo-Herrero says. “No
superconductivity or Mott insulator appears. So you have to be very precise
with the alignment angle.”
At 1.1 degrees — a rotation that
has been predicted to be a “magic angle” — the researchers found the graphene
superlattice electronically resembled a flat band structure, similar to a Mott
insulator, in which all electrons carry the same energy regardless of their
momentum.
“Imagine the momentum for a car is
mass times velocity,” Jarillo-Herrero says. “If you’re driving at 30 miles per
hour, you have a certain amount of kinetic energy. If you drive at 60 miles per
hour, you have much higher energy, and if you crash, you could deform a much
bigger object. This thing is saying, no matter if you go 30 or 60 or 100 miles
per hour, they would all have the same energy.”
“Current for free” “
For electrons, this means that,
even if they are occupying a half-filled energy band, one electron does not
have any more energy than any other electron, to enable it to move around in
that band. Therefore, even though such a half-filled band structure should act
like a conductor, it instead behaves as an insulator — and more precisely, a
Mott insulator.
This gave the team an idea: What if
they could add electrons to these Mott-like superlattices, similar to how
scientists doped Mott insulators with oxygen to turn them into superconductors?
Would graphene assume superconducting qualities in turn?
To find out, they applied a small
gate voltage to the “magic-angle graphene superlattice,” adding small amounts
of electrons to the structure. As a result, individual electrons bound together
with other electrons in graphene, allowing them to flow where before they could
not. Throughout, the researchers continued to measure the electrical resistance
of the material, and found that when they added a certain, small amount of
electrons, the electrical current flowed without dissipating energy — just like
a superconductor.
“You can flow current for free, no
energy wasted, and this is showing graphene can be a superconductor,”
Jarillo-Herrero says.
Perhaps more importantly, he says
the researchers are able to tune graphene to behave as an insulator or a
superconductor, and any phase in between, exhibiting all these diverse
properties in one single device. This is in contrast to other methods, in which
scientists have had to grow and manipulate hundreds of individual crystals,
each of which can be made to behave in just one electronic phase.
“Usually, you have to grow
different classes of materials to explore each phase,” Jarillo-Herrero says.
“We’re doing this in-situ, in one shot, in a purely carbon device. We
can explore all those physics in one device electrically, rather than having to
make hundreds of devices. It couldn’t get any simpler.”
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