Evidence Emerges for a Fractional Topological Insulator
In 2023, scientists discovered a peculiar quantum state in a slightly twisted bilayer of molybdenum ditelluride (MoTe2). In that state, known as a fractional Chern insulator, the material’s interior is insulating, and current flows along the bilayer’s edges in just one direction. This conduction happens even without an applied magnetic field and involves quasiparticles that have a fraction of an electron’s charge. Now, in a modified MoTe2 twisted bilayer, Xiaoyang Zhu at Columbia University and his colleagues have found experimental signatures of a related state, called a fractional topological insulator (FTI) [1]. One key difference between the two states is that the current in an FTI is expected to flow in both directions along the material’s edges. An FTI is long sought after partly because it could help physicists find other topological states for potential applications in quantum computing.
Zhu and his colleagues added electron vacancies, or holes, to a MoTe2 twisted bilayer until each unit cell of the moiré pattern formed by the two layers had, on average, four-thirds of a hole. They then applied a highly sensitive optical technique called pump–probe modulation spectroscopy, where the material is illuminated with one laser pulse to perturb it and a second pulse to measure how its optical properties change in response.
The researchers’ measurements revealed a fragile antiferromagnetic phase in which electron spins alternately point up and down perpendicular to the material. This behavior is unusual for moiré systems and matches theoretical predictions for an FTI formed of two interwoven fractional Chern insulators whose edge currents flow in opposite directions. The team suggests that confirming the FTI’s presence in this material will require further experiments.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- Y. Wang et al., “Candidate for a fractional topological insulator in twisted MoTe2,” Phys. Rev. X 16, 031009 (2026).