Electrodynamic response of the type-II Weyl semimetal YbMnBi2

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. Materials based on quasi-two-dimensional bismuth layers were recently designed and provide an arena for studying the interplay between anisotropic Dirac fermions, magnetism, and structural changes, allowing the formation of Weyl fermions in condensed matter. Here, we perform an optical investigation of YbMnBi2, a representative type-IIWeyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal EuMnBi2. Our comparative study allows us to disentangle the optical fingerprints of type-IIWeyl fermions, but also challenges the present theoretical understanding of their electrodynamic response.