MIT physicists have now detected the fractional quantum Hall effect in basic graphene, although it has often only been observed at very strong magnetic fields. Electrons (blue ball) in a graphene/hexagonal boron nitride (hBN) moire superlattice stacked five layers deep interact with one another and exhibit fractional charge behavior. Sampson Wilcox, RLE is credited.
An exotic electronic state observed by MIT physicists could enable more robust forms of quantum computing
Due to its solitary negative charge, the electron is the fundamental building block of electricity. This is the physics lesson we learn in high school, and the majority of natural materials exhibit this property.
However, electrons can split into smaller and smaller parts of themselves in extremely rare states of matter. This extremely uncommon occurrence, referred to as "fractional charge," may aid in the development of robust, fault-tolerant quantum computers provided the unusual electronic state can be contained and managed.
The "fractional quantum Hall effect" has only been observed a few times to date, and most of those observations were made in very high, meticulously maintained magnetic fields, according to researchers. The result in a material that did not require such strong magnetic manipulation has only lately been observed by scientists.
"This five-layer graphene is a material system where many good surprises happen," MIT assistant professor of physics Long Ju, one of the study's authors, said. Fractional charge is simply so unusual, and we can now achieve this phenomenon without the need for a magnetic field and with a far simpler apparatus. That is significant for basic physics in and of itself. Additionally, it might make it possible for quantum computing to develop into a more resilient technology.
Zhengguang Lu, the lead author, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Seo, Liang Fu, Kenji Watanabe, and Takashi Taniguchi from the National Institute for Materials Science in Japan are Ju's co-authors from MIT.
A Bizarre State
One example of the strange events that might occur when particles stop operating as individual units and start acting as a group is the fractional quantum Hall effect. In certain conditions, such as when electrons are slowed from their typically fast speed to a crawl that allows the particles to perceive and communicate with one another, this collective "correlated" behavior appears. Rare electronic states, such the ostensibly unconventional splitting of an electron's charge, can result from these interactions.
The fractional quantum Hall effect was first identified in gallium arsenide heterostructures in 1982. In these systems, a gas of electrons is confined in a two-dimensional plane and is subjected to high magnetic fields. Later, the team was awarded a Physics Nobel Prize for their discoveries.
According to Ju, "this unit charges interacting in a way to give something like fractional charge was very, very bizarre, so [the discovery] was a very big deal." "No theory predictions existed at the time, so everyone was surprised by the experiments."
By applying magnetic fields to slow down the material's electrons sufficiently for them to interact, those researchers were able to produce their ground-breaking findings. The fields they operated in were around ten times more powerful than those that normally run an MRI machine.
Researchers at the University of Washington announced the first proof of fractional charge in the absence of a magnetic field in August 2023. They saw this "anomalous" variation of the effect in molybdenum ditelluride, a twisted semiconductor. The material was created in a particular configuration that theorists believed would provide the material with an intrinsic magnetic field strong enough to drive electron fractionalization in the absence of external magnetic control.
The "no magnets" result paved the way for topological quantum computing, a more secure variation of quantum computing in which a qubit is given additional protection when performing a computation by the addition of topology, a property that endures even in the face of slight deformation or disturbance. This computational system is based on a superconductor plus the fractional quantum Hall effect. It was nearly hard to grasp that a strong magnetic field was required to obtain fractional charge, and that the superconductor would typically be destroyed by the same magnetic field. Here, the fractional charges would function as a qubit, which is a quantum computer's fundamental building block.
Making Steps
Ju and his colleagues also occurred to notice anomalous fractional charge in graphene that same month, despite the fact that no one had predicted that the material would show such an effect.
Ju's group has been investigating the electrical behavior of graphene, which has demonstrated remarkable features on its own. Ju's group has most recently investigated pentalayer graphene, which is composed of five graphene sheets stacked one on top of the other, resembling steps on a stairway. By employing Scotch tape to exfoliate graphite, one can produce the embedded pentalayer graphene structure. The electrons in the structure slow to a crawl at ultracold temperatures and interact differently than they would at higher temperatures. This is what happens when the structure is placed in a refrigerator.
The new work's calculations suggest that electrons could interact even more intensely if the pentalayer structure were aligned with hexagonal boron nitride (hBN), a material whose atomic structure is similar to graphene's but whose dimensions are slightly different. When the two materials come together, they should create a moiré superlattice, which is a complex atomic structure resembling a scaffold that has the potential to slow down electrons in ways that resemble magnetic fields.
Ju, who also happened to install a new dilution refrigerator in his MIT lab last summer, adds, "We did these calculations, then thought, let's go for it." The team planned to use the ultralow temperatures to cool materials and examine unusual electronic phenomena.
The scientists created two examples of the hybrid graphene structure by first removing graphene layers from a graphite block and then identifying the five-layered flakes in the steplike shape with optical instruments. Subsequently, they positioned a second hBN flake on top of the graphene framework and pressed the graphene flake onto it. In the end, they fitted the structure with electrodes and chilled it almost to the point of near-zero.
They began to see indications of fractional charge, where the voltage is equal to the current multiplied by a fractional number and a few basic physics constants, when they introduced a current to the material and monitored the voltage output.
First author Lu states, "We didn't recognize it at first the day we saw it." Then, as we understood this was actually significant, we began to yell. It was a very unexpected occasion.
Co-first author Han continues, "This was probably the first serious samples we put in the new fridge." "We examined carefully to make sure that what we were seeing was real once we had calmed down."
The group verified that the graphene structure did, in fact, display the fractional quantum anomalous Hall effect through additional study. This is the first instance of the effect in graphene.
According to Ju, graphene "can also be a superconductor." Therefore, two completely different effects could be present in the same substance, just next to one another. Many undesirable consequences when bridging graphene with other materials are avoided when graphene talks to graphene.
For the time being, the team is still searching multilayer graphene for further uncommon electronic states.
He states, "We are diving in to explore many basic physics ideas and applications." "We are aware that there will be more."
.jpg)
0 Comments