Quantum Hall effect gains a new twist in graphene moiré systems

Physicists have long been drawn to the nonlinear Hall effect: a subtle variant of the classical Hall effect, in which an electric voltage appears perpendicular to a current flowing through a material. Unlike its classical counterpart, the nonlinear version can arise even without breaking time-reversal symmetry, and its magnitude is tied to deep geometric properties of electron wave functions. So far, however, the behavior of the effect when a magnetic field is applied has remained poorly understood.