Weyl semimetals are quantum materials in which electrons behave as if they are massless, moving with a linear energy-momentum relationship similar to photons. These materials also host Weyl fermions with a built‑in chirality, meaning their spin and momentum are locked in either a left‑ or right‑handed configuration.

A distinctive feature of Weyl semimetals is the presence of Fermi arcs which are surface electronic states that connect projections of bulk Weyl nodes. Because these arcs inherit the chirality of the underlying Weyl fermions, their motion is directionally biased and highly sensitive to the surface environment. This makes them promising for surface‑state engineering in topological devices.

The researchers show that the surface of the Weyl semimetal Co₃Sn₂S₂ can generate a strong, tunable second‑order nonreciprocal electrical response, which depends sensitively on the surface termination and can be further controlled by adjusting the surface potential. Crucially, when the Fermi arcs undergo a Fermi arc Lifshitz transition, a change in how the arcs connect across the surface Brillouin zone, the nonlinear current reverses sign. This sign flip arises from the chiral nature of electron velocities on the arcs.

The work demonstrates that measuring nonreciprocal transport provides a direct and experimentally accessible fingerprint of Fermi arc topology, offering a practical route to track and control surface states in Weyl semimetals without relying on complex surface‑sensitive probes.

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Recent progress on correlated electron systems with strong spin–orbit coupling by Robert Schaffer, Eric Kin-Ho Lee, Bohm-Jung Yang and Yong Baek Kim (2016)