A team of US researchers has unveiled a device that can conduct electricity along its fractionally charged edges without losing energy to heat. Described in Nature Physics, the work, led by Xiaodong Xu at the University of Washington, marks the first demonstration of a “dissipationless fractional Chern insulator,” a long-sought state of matter with promising implications for future quantum technologies.
From quantum Hall to fractional phases
The quantum Hall effect emerges when electrons are confined to a two-dimensional material, cooled to extremely loow temperatures, and exposed to strong magnetic fields. Much like the classical Hall effect, it describes how a voltage develops across a material perpendicular to the direction of current flow. In this case, however, that voltage increases in discrete, or quantized steps.⁴
Under even more extreme conditions, an exotic variant appears named the “fractional quantum Hall” (FQH) effect. Here, electrons no longer behave as independent particles but move collectively, giving rise to voltage steps that correspond to fractions of an electron’s charge. This unusual collective behavior unlocks a whole host of exotic properties, and has made such states particularly appealing for emerging quantum technologies.
Bringing fractional behavior to zero field
An even stranger possibility arises in fractional Chern insulators (FCIs). These materials can exhibit a fractionally quantized Hall signal, just like the FQH effect, but at zero magnetic field. While FCIs were first proposed more than a decade ago, their defining behavior, known as the “fractional quantum anomalous Hall” (FQAH) effect, was demonstrated experimentally for the first time by Xu’s group in 2023. Here, the researchers used devices made from two layers of molybdenum ditelluride, twisted relative to one another at a carefully chosen angle.
“The initial discovery of the FQAH effect was exciting, but far from perfect,” Xu recounts. “For instance, although the Hall resistance was quantized at the expected value, the longitudinal resistance, which is expected to vanish, was still appreciable.”
Engineering cleaner, higher-quality devices
That lingering resistance pointed to energy dissipation, where electrical energy is lost as heat as charges move through the material. In their latest study, Xu and his colleagues tackled this problem by improving device performance in two key areas.
The first focused on the growth of the underlying crystals themselves. “My colleague Jiun-Haw Chu and our joint postdoc Chaowei Hu found that horizontal flux growth drastically improves crystal quality,” he says. “Compared with the crystals used in the original 2023 study, the new approach boosted charge-carrier mobility by more than an order of magnitude.”
“Second, my student Heonjoon Park, together with others, has further improved the device fabrication process to reduce twist-angle disorder.”
Reaching dissipationless edge conduction
With these improvements in place, the researchers observed that the unwanted resistance nearly vanished when the system was tuned to a state corresponding to two-thirds filling of the electronic band. The result represents the first realization of a dissipationless FCI, in which electrical energy flows along the edges with essentially no loss to heat.
The cleaner devices also revealed unexpected behavior in the system’s thermal activation gap: the energy separating the bulk ground state from its lowest excited states. If this gap is too small, thermally excited bulk electrons can compete with the edge states, degrading device performance.
A puzzling energy gap and future outlook
“To our surprise, we found that the thermal activation gap of the fractional state rapidly decreases as the magnetic field increases and then plateaus above a certain magnetic field strength,” Xu says. “This contrasts with FQH states, where a magnetic field is needed to form the state and further enhance the energy gap by increasing its strength.”
The team’s theory suggests that this unusual trend arises from a competition between different types of low-energy excitations tied to electron spin and charge, each requiring different amounts of energy to activate in the FQAH system.
Looking ahead, Xu and his colleagues see a clear path toward even better performance. “The quantum Hall community has made repeated breakthroughs by improving sample quality over the last 40 years,” he says. “Building on this experience, we hope that the progress on this new platform will be even faster, and we cannot wait to see what new surprises lie ahead.” https://phys.org/news/2026-02-current-loss-newly-fractional-quantum.html
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