Two-Dimensional Topological Weyl Fermion States in Epitaxial Atomic Layers

The low-energy properties of quantum materials are deeply connected to fundamental concepts in high-energy physics such as Weyl fermions, Majorana fermions, chiral anomalies, and supersymmetry. Recent advancements in synthesizing high-quality epitaxial atomic layers have opened up vast opportunities for exploring new fundamental physics in topological quantum materials. In this talk, we will report the realization of a 2D topological Weyl (half-Dirac) semimetal, which is a spin-polarized version of graphene. The nonzero winding number of spin-polarized Weyl bands guarantees the existence of topologically protected edge states, which take the form of Fermi strings with one end attached to Weyl nodes at the Fermi level. In this context, 2D Weyl semimetals present a new paradigm of bulk–boundary correspondence in topological materials. The 2D Weyl semimetal has been experimentally realized in epitaxial Bi monolayer grown on SnSe(S) substrates. The spin polarization of massless bulk states and topological Fermi string edge states have been observed by spin-ARPES and STS dI/dV measurements. The 2D Weyl fermion states and topological edge states offer fresh insights into the topological characteristics of relativistic electrons in reduced dimensions. We will discuss the exotic electromagnetic properties of 2D Weyl semimetals, including nonlinear Hall effects, topological currents, parity anomalies, and nonlocal transport phenomena.