Tuning Co₃Sn₂S₂’s surface switches the sign of its nonlinear transport Tangled colourful arrows (Courtesy: iStock/Adventtr) 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.…