Materials physics research has recently been captivated by a strange phenomenon where electrons split into parts of their charges in graphene. This discovery challenges our fundamental knowledge of electronic behaviors and could influence the development of future quantum devices. How do these electronic moieties emerge without the need for a magnetic field, and what are the implications for materials science?
MIT physicists have taken a decisive step toward solving the enigma of electron fractionalization. Their solution sheds light on the conditions allowing the emergence of exotic electronic states in graphene and other two-dimensional systems.
The recent study attempts to explain a discovery reported earlier this year by another group of MIT physicists, led by assistant professor Long Ju. Ju and his team observed that the electrons appear to show a “fractional load» in five-layer graphene, a configuration consisting of five layers of graphene superimposed on a similarly structured sheet of boron nitride.
Ju found that when an electric current passed through the five-layer structure, the electrons appeared to pass as fractions of their total charge, even in the absence of a magnetic field. Scientists had previously demonstrated that electrons could split into fractions under the influence of a very strong magnetic field, known as the fractional quantum Hall effect. Ju’s work was the first to prove that this effect was possible in graphene without a magnetic field, an unexpected phenomenon until recently.
The Anomalous Quantum Hall Effect Fractional Anomalous
This phenomenon has been called “ fractional anomalous quantum Hall effect ”, and theorists sought to explain how fractional charge can emerge from five-layer graphene.
The new study, led by MIT physics professor Senthil Todadri, provides part of the answer. Through calculations of quantum mechanical interactions, he and his colleagues show that electrons form a kind of crystal structure whose properties are ideal for allowing the emergence of electronic moieties.
« This is an entirely new mechanism, meaning that in decades-long history, people have never had a system that drives these fractional electronic phenomena. “, said Senthil Todadri. He added: “ This is really exciting because it makes all sorts of new experiences possible that one could only dream of before. »
A New Theoretical Framework
The MIT study was published last week in the journal Physical Review Letters. Two other research teams, one from Johns Hopkins University and another from Harvard, the University of California at Berkeley and the Lawrence Berkeley National Laboratory, published similar results in the same issue. The MIT team includes Zhihuan Dong, PhD ’24, and former postdoc Adarsh Patri.
In 2018, MIT physics professor Pablo Jarillo-Herrero and his colleagues were the first to observe that new electronic behaviors could emerge by stacking and twisting two sheets of graphene. This “magic angle graphene”, as it quickly became known, sparked a new area of research called twistronics, the study of electronic behavior in twisted two-dimensional materials.
« Shortly after his experiments, we realized that these moiré systems would be ideal platforms in general for finding the conditions that allow the emergence of these fractional electronic phases. “, explained Professor Todadri, who collaborated with Jarillo-Herrero on a study the same year to show that, in theory, such twisted systems could exhibit fractional charge without a magnetic field.
Experimental Surprises
In September 2023, Todadri had a conversation via Zoom with Ju, who was familiar with his theoretical work and had maintained contact through his own experimental research. Ju showed him data where he had observed these electronic fractions in five-layer graphene, which was a big surprise because it didn’t match initial predictions.
In his 2018 paper, Todadri predicted that fractional charge should emerge from a precursor phase characterized by a particular twist in the electronic wave function. He had theorized that the quantum properties of an electron should have a certain twist, or degree of manipulation possible without changing its inherent structure. This twist, he predicted, should increase with the number of graphene layers added to a given moiré structure.
« For five-layer graphene, we thought the wave function would twist five times, and that this would be a precursor to electronic fractions. “, said Todadri. “But he did his experiments and found that it twisted, but only once. This then raised a big question: how should we think about what we see? »
Illustration caption: An electron cloud crystal could explain the puzzling fractional charge recently discovered in multilayer graphene.
Article : « Theory of Quantum Anomalous Hall Phases in Pentalayer Rhombohedral Graphene Moiré Structures » – DOI: 1721.1/157541
Source: MIT
