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The fractional quantum Hall effect has generally been observed in very strong magnetic fields, but MIT physicists have now observed it in simple graphene. In a five-layer graphene/hexagonal boron nitride (hBN) moiré superlattice, electrons (blue ball) interact strongly with each other and behave as if they were split into fractional charges. Credit: Sampson Wilcox, RLE
An exotic electronic state observed by M.I.T. Physicists could allow for more robust forms of quantum computing.
The electron is the basic unit of electricity, as it carries a single negative charge. This is what we are taught in high school physics, and it is overwhelmingly the case in most materials in nature.
But in very special states of matter, electrons can fragment into fractions of their entirety. This phenomenon, known as “fractional charging,” is extremely rare, and if it can be corralled and controlled, the exotic electronic state could help build robust, fault-tolerant quantum computers.
To date, this effect, known to physicists as the “fractional quantum Hall effect,” has been observed several times, and most often under very high and carefully maintained magnetic fields. Only recently have scientists seen the effect in a material that did not require such powerful magnetic manipulation.
Now, MIT physicists have observed the elusive fractional charge effect, this time in a simpler material: five layers of graphene – a atom-thin layer of carbon that comes from graphite and common pencil lead. They report their results on February 21 in the magazine Nature.
A photo of the team. From left to right: Long Ju, postdoc Zhengguang Lu, visiting student Yuxuan Yao, graduate student Tonghang Hang. Credit: Jixiang Yang
They found that when five sheets of graphene are stacked like steps on a ladder, the resulting structure inherently provides the right conditions for electrons to pass through as fractions of their total charge, without the need for any external magnetic field.
The results are the first evidence of the “fractional quantum anomalous Hall effect” (the term “anomalous” refers to the absence of a magnetic field) in crystalline graphene, a material that physicists did not expect to exhibit this effect.
“This five-layer graphene is a material system in which many good surprises occur,” says study author Long Ju, an assistant professor of physics at MIT. “Fracial charging is very exotic and now we can achieve this effect with a much simpler system and without a magnetic field. This in itself is important for fundamental physics. And it could enable the possibility of a type of quantum computing that is more robust against perturbations.”
Ju’s co-authors at MIT are lead author Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Seo and Liang Fu, along with Kenji Watanabe and Takashi Taniguchi of Japan’s National Institute of Materials Science.
a strange state
The fractional quantum Hall effect is an example of the strange phenomena that can arise when particles go from behaving as individual units to acting together as a whole. This collective “correlated” behavior arises in special states, for example, when electrons slow down from their normally frenetic pace to a crawl that allows particles to sense each other and interact. These interactions can produce rare electronic states, such as the seemingly unorthodox splitting of an electron’s charge.
In 1982, scientists discovered the fractional quantum Hall effect in gallium arsenide heterostructures, where a gas of electrons confined in a two-dimensional plane is placed under strong magnetic fields. The discovery later earned the group the Nobel Prize in Physics.
“(The discovery) was very important, because these unit charges interacting in a way to give something like a fractional charge was very, very strange,” says Ju. “At that time there were no theoretical predictions and the experiments surprised everyone.”
Those researchers achieved groundbreaking results by using magnetic fields to slow down the material’s electrons enough so they could interact. The fields they worked with were about 10 times more powerful than those that normally power an MRI machine.
In August 2023, scientists at University of Washington reported the first evidence of fractional charging without a magnetic field. They observed this “anomalous” version of the effect, in a twisted semiconductor called molybdenum ditelluride. The group prepared the material in a specific configuration, which theorists predicted would give the material an inherent magnetic field, enough to encourage electrons to split without any external magnetic control.
The “no magnets” result opened a promising route to topological quantum computing, a more secure form of quantum computing, in which the added ingredient of topology (a property that remains unchanged under a weak deformation or perturbation) provides additional protection to the qubit. when performing a calculation. This calculation scheme is based on a combination of the fractional quantum Hall effect and a superconductor. Before it was almost impossible to realize: a strong magnetic field is needed to obtain a fractional charge, while the same magnetic field normally kills the superconductor. In this case, the fractional charges would serve as a qubit (the basic unit of a quantum computer).
taking steps
That same month, Ju and his team also observed signs of anomalous fractional charge in graphene, a material for which there had been no predictions to exhibit such an effect.
Ju’s group has been exploring the electronic behavior of graphene, which itself has exhibited exceptional properties. More recently, Ju’s group has studied pentalayer graphene, a structure of five sheets of graphene, each stacked slightly apart from the other, like steps on a ladder. This pentalayer graphene structure is embedded in graphite and can be obtained by peeling with adhesive tape. When placed in a refrigerator at ultracold temperatures, the electrons in the structure slow down and interact in a way they normally would not when spinning at higher temperatures.
In their new work, the researchers did some calculations and discovered that electrons could interact with each other even more strongly if the pentalayer structure was lined with hexagonal boron nitride (hBN), a material that has a similar atomic structure to that of graphene. but with slightly different dimensions. In combination, the two materials should produce a moiré superlattice, an intricate scaffold-like atomic structure that could slow down electrons in ways that mimic a magnetic field.
“We did these calculations and then we thought, let’s do it,” says Ju, who installed a new dilution refrigerator in his MIT lab last summer, which the team planned to use to cool materials to ultra-low temperatures, to study exotic materials. electronic behavior.
The researchers fabricated two samples of the hybrid graphene structure by first exfoliating graphene layers from a block of graphite and then using optical tools to identify five-layer flakes in the staggered configuration. They then stamped the graphene flake onto an hBN flake and placed a second hBN flake on top of the graphene structure. Finally, they connected electrodes to the structure and placed it in the refrigerator, at a temperature close to Absolute zero.
When they applied a current to the material and measured the voltage output, they began to see signatures of fractional charge, where the voltage is equal to the current multiplied by a fractional number and some fundamental physical constants.
“The day we saw it, we didn’t recognize it at first,” says first author Lu. “Then we started screaming when we realized this was really big. “It was a completely surprising moment.”
“This was probably the first serious sample we put in the new refrigerator,” adds co-author Han. “Once we calmed down, we looked closely to make sure what we were seeing was real.”
With more detailed analysis, the team confirmed that the graphene structure did indeed exhibit the fractional quantum anomalous Hall effect. It is the first time that the effect has been observed in graphene.
“Graphene can also be a superconductor,” says Ju. “So you could have two totally different effects on the same material, side by side. “If you use graphene to talk to graphene, you avoid a lot of unwanted effects of bonding graphene to other materials.”
For now, the group continues to explore multilayer graphene for other rare electronic states.
“We are diving into the exploration of many fundamental ideas and applications of physics,” he says. “We know there will be more to come.”
Reference: “Fractional quantum anomalous Hall effect in multilayer graphene” by Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan P. Reddy, Jixiang Yang, Junseok Seo, Kenji Watanabe, Takashi Taniguchi, Liang Fu and Long Ju, February 21, 2024 , Nature.
DOI: 10.1038/s41586-023-07010-7
This research is supported in part by the Sloan Foundation and the National Science Foundation.