US Scientists Achieve First Dissipationless Fractional Chern Insulator, Opening New Era for Quantum Devices
A team of researchers in the United States has revealed a device capable of carrying electrical current along its fractionally charged edges without wasting energy as heat. Reported in Nature Physics, the breakthrough—led by Xiaodong Xu at the University of Washington—represents the first experimental realization of a dissipationless fractional Chern insulator, a long-theorized state of matter with major potential for next-generation quantum technologies.
Understanding the Quantum Hall Effect
The quantum Hall effect arises when electrons are confined within a two-dimensional material, cooled to near absolute zero and subjected to intense magnetic fields. As in the classical Hall effect, a voltage forms at right angles to the flow of current—but in this quantum version, the voltage increases in precise, quantized steps.
Fractional Quantum Hall Effect and Collective Electron Behaviour
Pushing these conditions even further reveals a more exotic phenomenon known as the fractional quantum Hall effect (FQH). In this regime, electrons cease to act as individual particles and instead move collectively, producing voltage steps that reflect fractional units of an electron's charge.
This striking behaviour gives rise to a range of unusual properties, making such states highly attractive for next-generation quantum technologies.
Fractional Chern Insulators: Quantum States Without Magnetic Fields
An even more unusual scenario emerges in fractional Chern insulators, or FCIs. These materials can display a fractionally quantized Hall signal—much like the fractional quantum Hall effect (FQH)—but without the need for an external magnetic field.
Although FCIs were first proposed over a decade ago, their hallmark behaviour, known as the fractional quantum anomalous Hall (FQAH) effect, was demonstrated experimentally for the first time in 2023 by Xiaodong Xu's team. The breakthrough relied on devices built from two layers of molybdenum ditelluride, carefully twisted relative to one another.
Early Limitations in the First FQAH Devices
"The initial observation of the FQAH effect was thrilling, but it was not yet ideal," Xu explains.
"While the Hall resistance appeared at the correct quantized value, the longitudinal resistance—which should disappear—remained noticeably large."
The persistent resistance signaled ongoing energy dissipation, with electrical power being lost as heat as charges travelled through the material.
Eliminating Energy Loss: Two Key Experimental Advances
In their latest work, Xu and his team set out to eliminate this loss by refining the device across two critical fronts.
Improved Crystal Growth Techniques
The first improvement centered on the growth of the crystals themselves. Xu explains that his colleague Jiun-Haw Chu, working alongside postdoctoral researcher Chaowei Hu, discovered that horizontal flux growth dramatically enhanced crystal quality.
Compared with the materials used in the 2023 study, the new crystals delivered more than a tenfold increase in charge-carrier mobility.
Reduced Disorder in Device Fabrication
The second advance came from further refinements in device fabrication. Xu notes that his student Heonjoon Park and collaborators successfully reduced disorder in the twist angle, a key factor limiting performance.
First Experimental Realization of a Dissipationless Fractional Chern Insulator
With these refinements in place, the team found that the unwanted resistance almost completely disappeared when the system was tuned to a two-third filling of the electronic band.
This milestone marks the first experimental realization of a dissipationless fractional Chern insulator, where electrical current flows along the edges with virtually no energy lost as heat.
Unexpected Behaviour in the Thermal Activation Gap
The improved device quality also uncovered unexpected features in the system's thermal activation gap —the energy difference the bulk ground state and its lowest excited states.
If this gap becomes too small, thermally excited electrons in the bulk can interfere with edge transport, undermining overall performance.
Xu says the researchers were surprised to find that the thermal activation gap of the fractional state shrinks quickly as the magnetic field increases, before flattening out beyond a threshold.
"This stands in stark contrast to fractional quantum Hall states, where the magnetic field both creates the state and strengthens it by widening the energy gap," he notes.
Theoretical Explanation Behind the Anomaly
According to the team's theoretical model, the unusual behaviour stems from a tug-of-war between different low-energy excitations associated with electron spin and charge, each requiring a different amount of energy to emerge in the FQAH regime.
Implications for Future Quantum Technologies
The researchers believe this is only the beginning. Xu points to the long history of breakthroughs driven by improved sample quality in quantum Hall research and says similar progress could unfold even faster on this emerging platform.
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