Fermi-arc-induced chiral transport in Weyl semimetal TaAs

Topological Weyl semimetals host unclosed surface states, known as Fermi arcs, that connect bulk Weyl nodes in momentum space. Both bulk Weyl nodes and Fermi arcs are anticipated to be chiral. The chirality of bulk bands has been confirmed through observations of the chiral anomaly and Weyl orbits. In contrast, despite their discovery more than a decade ago, the chiral nature of Fermi arcs has remained unresolved. Here we report Fermi-arc-induced chiral transport in the archetypal Weyl semimetal TaAs. Using focused ion beam techniques, we fabricated micro-sized devices that enable simultaneous transport measurements on opposing topological surfaces. Linear transport is dominated by bulk conduction, whereas nonlinear transport reveals distinct surface contributions with opposite chirality on the top and bottom surfaces. Angle-resolved measurements further uncover an exceptionally large third-order nonreciprocal response that far exceeds conventional transport predictions, highlighting the crucial role of the singular arc endpoints. Our findings unambiguously demonstrate the chiral nature of Fermi arcs and establish nonlinear transport as a direct probe of these topological surface states. By revealing a surface-switchable, room-temperature nonlinear response that is topologically protected, this work establishes a new functionality in Weyl semimetals. Given that numerous natural materials predicted to host topological semimetal states, these results open opportunities for exploring nonlinear transport phenomena and potential device concepts across a broad class of systems.