MIT physicists have advanced our understanding of electron behavior, indicating that these subatomic particles can split into fractions of themselves. This insight deepens our knowledge of graphene, a material comprising a one-atom-thick layer of carbon, known for its exceptional strength and conductivity. As reported byMIT News, the study zeroes in on the so-called “fractional quantum anomalous Hall effect” observed in pentalayer graphene. This effect, previously unanticipated in graphene absent a magnetic field, is shedding light on possible exotic electronic states within two-dimensional structures.
The original discovery, having been made by Assistant Professor Long Ju and his team earlier in 2024, showed fractions of electron charges moving through graphene, fundamentally changing the way scientists understand electron behavior in such materials. Ju stumbled across this behavior while running an electric current through pentalayer graphene atop a boron nitride structure. Expected to be caused by a robust magnetic field through the fractional quantum Hall effect, these fractional charges emerged without it, setting the stage for new theories to explain the phenomenon.
Much of the groundbreaking work explaining this behavior has come from MIT professor of physics Senthil Todadri, who alongside his team, conducted quantum mechanical calculations that revealed electrons form a crystal structure in these conditions. Todadri noted in his team’s findings, published in Physical Review Letters, that “This is a completely new mechanism, meaning in the decades-long history, people have never had a system go toward these kinds of fractional electron phenomena.” His statement, obtained byMIT News, underlines the groundbreaking nature of their work.
The leap in understanding owes much to the phenomenon of moir patterns, resulting from the stacking of two-dimensional materials like graphene, and first brought to prominence by MIT professor Pablo Jarillo-Herrero’s research in 2018. When two sheets of graphene are aligned and twisted at a very specific angle, unexpected and unordinary properties become apparent, such as showing superconducting and insulating states simultaneously. It was Todadri who, collaborating with Jarillo-Herrero, speculated that such twisted moir systems might be the ideal grounds for observing fractional electron charges absent a magnetic field.
However, when Ju shared his experimental findings showing that electron fractions were actually observed in pentalayer graphene, it unveiled an unexpected twist in Todadri’s theoretical predictions. He initially believed that a specific twisting of the electron wavefunction would be the precursor to these fractions, but Ju’s experiments proved otherwise. According toMIT News, Todadri’s revision of his hypothesis considered the interelectron interactions, which had been somewhat overlooked before. In two-dimensional confines like pentalayer graphene, electrons are forced to obey quantum correlations, not just repulsion, which led to a theoretical prediction concurrent with Ju’s observations.
Building upon their matching theory and observation, Todadri’s team uncovered the process by which pentalayer graphene encourages fractional charge. They found that the electron crystal formed under the weak electrical potential of the moir pattern engages in a tug-of-war. Here, interelectron quantum correlations build a cloud of potential physical states, ultimately leading to the emergence of fractional charges. This mechanism, laying the foundation for understanding such observations in pentalayer graphene, also opens the door for predicting similar behavior in other two-dimensional systems.
Todadri’s work, described by his team in the Physical Review Letters, is fueling future research not only to understand this extraordinary crystal but also to probe other properties that may arise from such unprecedented electronic states. This research has been partly supported by entities including the National Science Foundation and the Simons Foundation, signaling the large-scale academic interest vested in the potential of graphene and related materials to revolutionize electronics.
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