Discovery of a Two-Dimensional Weyl Semimetal

The discoveries of Dirac semimetals in graphene and bulk Weyl semimetals in TaAs have led to an exciting territory of modern condensed matter physics where many emergent phases are generated from the interplay of quantum mechanics, relativity, and topology. So far, a two-dimensional (2D) variant of Weyl semimetals remains elusive for the instability of linear band crossings in reduced dimensions. In this work, we report the realization of a 2D Weyl semimetal in epitaxial bismuthene film. The Weyl fermion states originated from the gapped Dirac cones of bismuthene under the influence of symmetry-breaking substrate effects. Our high-resolution photoemission measurement shows a gapless dispersion and a clean Fermi surface. The spin polarization of the Weyl fermion states is observed in spin-resolved photoemission measurements. The bulk band topology of Weyl cones guarantees the existence of a new type of boundary mode, which is a 1D analog of Fermi arc states. These results establish epitaxial bismuthene as an ideal 2D Weyl semimetal that provides a unique platform for the experimental study of Weyl fermion states in low dimensions.