Twisted bilayer graphene has unexpectedly exhibited quantum oscillations that defy the established rules of the linear Hall effect. Chinese physicists observed these oscillations directly in an experiment, demonstrating that they originate from the system’s nonlinear response.
A research team led by Junxi Duan at the Beijing Institute of Technology performed measurements on samples of small-angle twisted bilayer graphene. The scientists applied an alternating electric field and recorded a transverse current that fluctuated with a periodicity determined by the magnetic field. Their findings were published in Physical Review Letters in June 2026.
The conventional Hall effect produces a linear response where the current is proportional to the field. In this case, however, a quadratic term appeared, generating oscillations similar to the Shubnikov–de Haas effect but occurring without the typical filling of Landau levels. Imagine a pendulum that swings not just from a push, but because the push itself distorts the trajectory—this is how nonlinearity "drives" the quantum states.
These oscillations allow researchers to probe the quantum geometry of electron wave functions beyond the linear approximation. They are directly linked to the topological properties and strong electronic interactions found within moiré graphene systems.
The discovery provides a new tool for investigating topological phases and interacting states, which are fundamental to the development of future quantum materials. It is now possible to measure nonlinear contributions in real-time without damaging the sample.
The results, appearing in Physical Review Letters, confirm that the nonlinear Hall effect in twisted graphene opens a window into a previously inaccessible realm of quantum geometry.
