New graphene breakthrough from MIT shapes the future of quantum computing

The partial quantum Hall effect has generally been seen under very high magnetic fields, but MIT physicists have now observed it in simple graphene. In a five-layer superlattice of graphene/hexagonal boron nitride (hBN), electrons (blue ball) interact strongly with each other and behave as if they were divided into fractional charges. Credit: Sampson Wilcox, RLE

Strange electronic condition noticed Massachusetts Institute of Technology Physicists can enable more powerful forms of… Quantitative statistics.

The electron is the basic unit of electricity, because it carries a single negative charge. This is what we learned in high school physics, and this is overwhelmingly the case in most subjects in nature.

But in very special states of matter, electrons can split into parts of their entire total. This phenomenon, known as “partial charge,” is extremely rare, and if it can be trapped and controlled, the exotic electronic state could help build flexible, fault-tolerant quantum computers.

So far, this effect, known to physicists as the “fractional quantum Hall effect,” has been observed many times, mostly under very high and carefully maintained magnetic fields. Only recently have scientists discovered the effect in a material that does not require such strong magnetic manipulation.

Now, physicists from MIT have observed the elusive partial charge effect, this time in a simpler material: five layers of… Graphene – that corn– A thin layer of carbon originates from graphite and ordinary lead. They reported their findings on February 21 in the journal nature.

Graphene research team to break electrons

Team photo. From left to right: Long Ju, postdoctoral researcher Zhengguang Lu, visiting undergraduate student Yuxuan Yao, graduate student Tonghang Huang. Credit: Jixiang Yang

They found that when five sheets of graphene are stacked like the rungs of a ladder, the resulting structure inherently provides the right conditions for electrons to pass through as parts of their overall charge, without the need for any external magnetic field.

The results are the first evidence of a “partial quantum anomalous Hall effect” (“anomalous” refers to the absence of a magnetic field) in crystalline graphene, a material that physicists did not expect to exhibit this effect.

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“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. “Fractional charge is very strange, and now we can achieve this effect using a much simpler system and without a magnetic field. This in itself is important for fundamental physics. It could open up the possibility of a type of quantum computing that is more robust against disturbance.”

Co-authors at MIT include lead author Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Seo, and Liang Fu, along with Kenji Watanabe and Takashi Taniguchi at the National Institute of Materials Science in Japan.

Strange country

The partial quantum Hall effect is an example of the strange phenomena that can arise when particles switch from behaving as individual units to behaving together as a whole. This collective “coherent” behavior appears in special cases, for example when electrons are slowed from their normally frenetic speed to a crawl that enables molecules to sense each other and interact. These interactions can produce rare electronic states, such as unconventional splitting of the electron charge.

In 1982, scientists discovered the partial quantum Hall effect in gallium arsenide heterostructures, in which a gas of electrons confined in a two-dimensional plane is held under high magnetic fields. This discovery later led to the group receiving the Nobel Prize in Physics.

“[The discovery] “This was a very big problem, because the interaction of these charge units in a way that gave something like a fractional charge was very strange,” says Joe. “At that time, there were no theoretical predictions, and the experiments surprised everyone.”

These researchers achieved their pioneering results by using magnetic fields to slow down a material's electrons enough for them to interact. The fields they worked with were about 10 times stronger than those that normally power an MRI machine.

In August 2023, scientists at University of Washington He reported the first evidence of the existence of partial charge without a magnetic field. They observed this “anomalous” version of the effect, in a twisted semiconductor called molybdenum ditelluride. The group prepared the material with a specific configuration, which theorists predicted would give the material an inherent magnetic field, enough to encourage electrons to split apart without any external magnetic control.

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The “no magnet” result has opened a promising path to topological quantum computing – a safer form of quantum computing, where the additional component of topology (a property that remains unchanged in the face of distortion or weak disturbance) provides additional protection for the qubit when performing a calculation. This calculation scheme is based on a combination of partial quantum Hall effect and superconductivity. It was almost impossible to realize this: one needs a strong magnetic field to get a partial charge, while the same magnetic field would usually kill a superconductor. In this case, the fractional charges would be a qubit (the basic unit of a quantum computer).

Making steps

In the same month, Gu and his team also happened to notice signs of an anomalous partial charge in graphene, a material that was not expected to show such an effect.

Gu's group has been exploring electronic behavior in graphene, which itself has demonstrated exceptional properties. Recently, Gu's group investigated pentalayer graphene, a structure made up of five graphene sheets, each stacked slightly apart from the others, like the rungs of a ladder. This pentagonal graphene structure is embedded in graphite and can be obtained by exfoliation with Scotch tape. When placed in a freezer at very cold temperatures, the structure's electrons slow to a crawl and react in ways they normally wouldn't when roaming around at higher temperatures.

In their new work, the researchers performed some calculations and found that the electrons might interact with each other more strongly if the pentagonal layer structure was aligned with hexagonal boron nitride (hBN) – a material with an atomic structure similar to that of graphene, but with slightly different dimensions. Combined, the two materials should produce a superlattice, a complex scaffold-like atomic structure that can slow the movement of electrons in ways that mimic a magnetic field.

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“We did these calculations, and then we thought, 'Let's do it,'” says Joe, who happened to have installed a new dilution refrigerator in his lab at MIT last summer, which the team planned to use to cool materials to extremely low temperatures. Electronic behavior.

The researchers fabricated two samples of the hybrid graphene structure by first peeling off layers of graphene from a block of graphite, then using optical tools to identify the five-layer flakes in a graded configuration. They then stamped the graphene wafer onto an hBN wafer and placed a second hBN wafer on top of the graphene structure. Finally, they attached electrodes to the structure and placed it in a freezer, then placed it in close proximity Absolute zero.

When they applied a current to the material and measured the voltage output, they began to see the signatures of fractional charge, where voltage equals current multiplied by a fractional number and some basic physical constants.

“The day we saw him, we didn't recognize him at first,” says first author Lu. “Then we started screaming when we realized this was a really big deal. It was a completely surprising moment.”

“These were probably the first serious samples we put in the new refrigerator,” adds co-first author Hahn. Once we calmed down, we looked into the details to make sure what we were seeing was real.”

With further analysis, the team confirmed that the graphene structure did indeed exhibit a partial quantum anomalous Hall effect. This is the first time this effect has been shown in graphene.

“Graphene could also be a superconductor,” says Gu. “So, you can have two completely different effects in the same material, next to each other. If you use graphene to talk to graphene, it avoids a lot of unwanted effects when you bond graphene to other materials.”

Currently, the team continues to explore multilayer graphene for other rare electronic states.

“We dive in to explore many fundamental physics ideas and applications,” he says. “We know there will be more to come.”

Reference: “Partial 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, 21 February 2024, nature.
doi: 10.1038/s41586-023-07010-7

This research is supported in part by the Sloan Foundation and the National Science Foundation.

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