  A two-dimensional (2D) Weyl semimetal featuring a spin-polarized linear band
dispersion and a nodal Fermi surface is a new topological phase of matter. It
is a solid-state realization of Weyl fermions in an intrinsic 2D system. The
nontrivial topology of 2D Weyl cones guarantees the existence of a new form of
topologically protected boundary states, Fermi string edge states. In this
work, we report the realization of a 2D Weyl semimetal in monolayer-thick
epitaxial bismuthene grown on SnS(Se) substrate. The intrinsic band gap of
bismuthene is eliminated by the space-inversion-symmetry-breaking substrate
perturbations, resulting in a gapless spin-polarized Weyl band dispersion. The
linear dispersion and spin polarization of the Weyl fermion states are observed
in our spin and angle-resolved photoemission measurements. In addition, the
scanning tunneling microscopy/spectroscopy reveals a pronounced local density
of states at the edge, suggesting the existence of Fermi string edge states.
These results open the door for the experimental exploration of the exotic
properties of Weyl fermion states in reduced dimensions.
