A newly-discovered class of quasiparticles known as fractional excitons offers fresh opportunities for condensed-matter research and could reveal unprecedented quantum phases, say physicists at Brown University in the US. The new quasiparticles, which are neither bosons nor fermions and carry no charge, could have applications in quantum computing and sensing, they say.
In our everyday, three-dimensional world, particles are classified as either fermions or bosons. Fermions such as electrons follow the Pauli exclusion principle, which prevents them from occupying the same quantum state. This property underpins phenomena like the structure of atoms and the behaviour of metals and insulators. Bosons, on the other hand, can occupy the same state, allowing for effects like superconductivity and superfluidity.
Fractional excitons defy this traditional classification, says Jia Leo Li, who led the research. Their properties lie somewhere in between those of fermions and bosons, making them more akin to anyons, which are particles that exist only in two-dimensional systems. But that’s only one aspect of their unusual nature, Li adds. “Unlike typical anyons, which carry a fractional charge of an electron, fractional excitons are neutral particles, representing a distinct type of quantum entity,” he says.
The experiment
Li and colleagues created the fractional excitons using two sheets of graphene – a form of carbon just one atom thick – separated by a layer of another two-dimensional material, hexagonal boron nitride. This layered setup allowed them to precisely control the movement of electrons and positively-charged “holes” and thus to generate excitons, which are pairs of electrons and holes that behave like single particles.
The team then applied a 12 T magnetic field to their bilayer structure. This strong field caused the electrons in the graphene to split into fractional charges – a well-known phenomenon that occurs in the fractional quantum Hall effect. “Here, strong magnetic fields create Landau electronic levels that induce particles with fractional charges,” Li explains. “The bilayer structure facilitates pairing between these positive and negative charges, making fractional excitons possible.”
“Distinct from any known particles”
The fractional excitons represent a quantum system of neutral particles that obey fractional quantum statistics, interact via dipolar forces and are distinct from any known particles, Li tells Physics World. He adds that his team’s study, which is detailed in Nature, builds on prior works that predicted the existence of excitons in the fractional quantum Hall effect (see, for example, Nature Physics 13, 751 2017, Nature Physics 15, 898-903 2019, Science 375 (6577), 205-209 2022).
The researchers now plan to explore the properties of fractional excitons further. “Our key objectives include measuring the fractional charge of the constituent particles and confirming their anyonic statistics,” Li explains. Studies of this nature could shed light on how fractional excitons interact and flow, potentially revealing new quantum phases, he adds.
“Such insights could have profound implications for quantum technologies, including ultra-sensitive sensors and robust quantum computing platforms,” Li says. “As research progresses, fractional excitons may redefine the boundaries of condensed-matter physics and applied quantum science.”
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