Scientists have observed a giant nonlinear Hall effect in bilayer graphene with broken isospin symmetry. This phenomenon, which manifests as a non-reciprocal electrical response, is significantly larger than previously reported nonlinear Hall effects in other materials. The research opens new avenues for the development of electronic devices based on the topological and symmetry properties of materials.

The nonlinear Hall effect arises from the interaction between electrons and defects or impurities in a material, or from the Berry curvature in momentum space, which generates a transverse current to the applied electric field even in the absence of a magnetic field. In this study, breaking the isospin symmetry in bilayer graphene, achieved by applying a perpendicular electric field, drastically amplified this effect. Isospin symmetry refers to a quantum property analogous to spin, but related to the valley degrees of freedom in graphene.

The researchers used a twisted bilayer graphene configuration, where the alignment of the layers is crucial for the electronic properties. By applying a displacement field, they managed to induce a valley polarization that breaks isospin symmetry, which in turn boosted the nonlinear Hall response. The magnitude of the observed effect is several orders of magnitude higher than in other known systems, making it a promising candidate for applications in low-power electronics and neuromorphic computing devices. This advance underscores the importance of symmetry engineering in quantum materials to unveil novel and exploitable physical phenomena.