Electrodynamic response of the type II Weyl semimetal YbMnBi2
Since the discovery of Dirac states in a wide range of materials spanning novel superconductors, graphene as well as topological insulators, a great deal of effort has been devoted to observe other types of elementary particles in condensed matter, like Majorana and Weyl fermions. The latter type of fermions may be understood as a pair of particles characterized by opposite chirality, as derived by the massless solution of the Dirac equation. We were triggered by the opportunity to exploit YbMnBi2 and EuMnBi2 as an arena in order to explore the optical response and chase the related fingerprints of a type II Weyl semimetal (i.e., in the Yb-based material) in contrast to its semimetal counterpart (i.e., the Eu compound).
Our optical experiment (Fig. 1.11) provides evidence for two intervals with a linear frequency dependence of the real part (σ1(ω)) of the optical conductivity in the Yb material, with the slope of the low-energy larger than the one of the high-energy interval. Both linear frequency dependences of σ1(ω) extrapolate to zero conductivity at the origin of the frequency axis. These features together with characteristic van Hove singularities are the major optical signatures of Weyl fermions. Indeed, in the Eu compound only one linear frequency dependence can be clearly identified at high frequencies, which cuts the frequency axis at a finite value and thus indicates its gapped nature. Overall, our comparative study broadly images the theoretical expectations within a minimum, simplified scenario tailored for type I Weyl fermions, but also calls for its extension to the case of type II Weyl semimetals, where the tilting of the single Dirac cone and the presence of broken time reversal symmetry are taken into account.