Fig. 2: Fully helicity-selective surface arcs.

Band dispersions measured on a W(110) and b 2 MLs Fe/W(110) along \(\overline{{{{{{{{{{\rm{H}}}}}}}}}}}-\overline{{{\Gamma }}}-\overline{{{{{{{{{{\rm{H}}}}}}}}}}}\) at the photon energy hν = 50 eV for the magnetization direction M+y. c Schematic diagram of the two co-propagating surface Fermi arcs (red lines) in the 2D topological ferromagnet, which appear inside the spin-orbit-induced projected band gap (large white area) of the substrate. d The measured circular dichroism reveals that the surface arcs in 2 MLs Fe/W(110) are 100% helicity-selective, with opposite helicity between the two sides of the momentum map. The labels RC and LC denote right and left circular polarization of the light, respectively, and the colour code represents the magnitude of the circular dichroism signal.