When mobile charge carriers, also known as itinerant electrons, interact with the strong exchange magnetic fields associated with the intrinsic angular momentum of localized electrons, this can give rise to the so-called Kondo effect. A Kondo insulator is a state of matter with an energy gap opened by the Kondo effect that forbids electrical conduction at low temperatures.
Like Kondo insulators, topological Kondo insulators are materials that behave as insulators (i.e., not conducting electricity) in their interior, but, unlike their counterparts without topology, can conduct electricity at their surface or edges. This unique, quantum phase of matter is protected by a material's internal symmetry and topology; thus, it is not easily disrupted.
So far, hints of this phase have been primarily observed in 3D quantum materials, such as samarium hexaboride (SmB₆) and ytterbium dodecaboride. Some physicists and material scientists have also been exploring the possible existence of this phase in 2D structures comprised of two materials stacked with a slight mismatch between them, producing a pattern known as a moiré superlattice.
Researchers at Cornell University, Max Planck Institute for the Structure and Dynamics of Matter, and the National Institute for Materials Science in Japan recently realized a 2D topological Kondo insulator based on a moiré bilayer structure, created by stacking thin molybdenum ditelluride (MoTe2) and tungsten diselenide (WSe2) layers.
Their paper, published in Nature Physics, opens new possibilities for the study of this rare, topology-protected quantum phase in 2D systems.
"A topological Kondo insulator is an exotic form of matter, in which electronic topology and correlation intersect," Kin Fai Mak, senior author of the paper, told Phys.org.
"Only a glimpse of its existence has been reported in 3D quantum materials, but not in the 2D flatland. Our objective is to realize this state of matter in 2D via moiré engineering, a platform for controlling quantum matter with unprecedented precisions."
Engineering and examining a 2D moiré bilayer system
This recent study builds on earlier papers by Mak and his colleagues, which focused on heavy fermion physics in moiré superlattices. Their previous findings inspired the team to explore the possibility that these materials could be tuned to become 2D topological Kondo insulators.
"The demonstration of a 2D topological Kondo insulator is not an easy task," explained Mak. "It requires several pieces of evidence, including that the material is a charge insulator in the bulk, as well as evidence of nearly quantized electrical transport from 1D edge channels and of the topological protection of these channels."
The researchers created moiré superlattices by stacking ultra-thin layers of MoTe2 and WSe2 on top of each other with a slight lattice mismatch between them. The researchers carefully engineered one layer to host localized magnetic moments and the other mobile electrons.
To demonstrate that the system they engineered hosts a 2D topological Kondo insulator phase, the researchers conducted several measurements.
"Firstly, we demonstrated that the bulk of the material (the 2D plane in this case) is a charge insulator by compressibility measurements and by bulk resistance measurements," said Mak.
"We then performed nonlocal transport measurements to detect nearly quantized electrical transport from the 1D edge channels surrounding the bulk. We also demonstrated the breakdown of the conducting 1D edges by an in-plane magnetic field, which indicates that topology protects the 1D edge channels. Finally, we broke down the 2D topological insulator by dissociating the Kondo singlets."
Overall, the measurements performed by Mak and his colleagues suggest that the material they engineered is in fact a 2D topological Kondo insulator. Their work therefore highlights the potential of moiré bilayer systems for studying strong interactions and topology-protected quantum phases of matter.
"Most topological insulators come from single-particle physics; thus, electron-electron interactions are not essential," said Mak. "In contrast, both single-particle band topology and electron-electron interactions are crucial to topological Kondo insulators.
"From a fundamental physics perspective, our work reports a rare scenario that topology arises from electron-electron interactions; it also paves the path for realizing other exotic states of matter, such as topological semimetals by doping a topological Kondo insulator."
The team's plans for further research
The researchers are now planning to continue investigating the bilayer system they engineered, to shed new light on its underlying physics. In addition, they would like to dope the material and explore the effects of this doping.
"It is known that an extremely rich electronic phase diagram can emerge by doping a correlated insulator (e.g. a Mott insulator)," said Mak. "The correlated insulator also carries topology in this case. It would be interesting to explore what will happen with this additional ingredient. The second direction is to explore possible unconventional quantum oscillations in the topological Kondo insulator."
Tiny and periodic variations in the measurable properties of materials under a strong magnetic field and at low temperatures, also known as quantum oscillations, generally only occur in metals and are not observed in insulators.
Some Kondo insulators, however, have been found to exhibit quantum oscillations under a magnetic field. As part of their next studies, the researchers hope to shed new light on the mechanisms behind this phenomenon.
"The ability to control our 2D topological Kondo insulator by gating may shed new light onto the origin of these mysterious quantum oscillations, if they emerge," added Mak.
"Lastly, we will explore the physics of the topological insulator when it is pushed away from the Kondo lattice regime and into a regime called the mixed-valence regime, where significant quantum fluctuations in the local magnetic moments could drive new physics."
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Zhongdong Han et al, Topological Kondo insulator in MoTe2/WSe2 moiré bilayers, Nature Physics (2026). DOI: 10.1038/s41567-026-03170-1.
Citation: 2D topological Kondo insulator observed in a moiré superlattice (2026, March 9) retrieved 10 March 2026 from https://phys.org/news/2026-03-2d-topological-kondo-insulator-moir.html
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