Condensed-Matter Physics & Materials Science Seminar

We will discuss the effects of short-range disorder on three-dimensional Weyl semimetals with a focus on the topological Fermi arc surface states and the existence of the axial anomaly in the presence of parallel electric and magnetic fields. We will briefly review the bulk properties of disordered Weyl semimetals concentrating on the proposed quantum critical point separating a semimetal and diffusive metal phase driven by disorder. We show that quasi-localized, rare eigenstates contribute an exponentially small but non-zero density of states at the Weyl node energy. This destabilizes the semimetal phase and converts the semimetal-to-diffusive metal transition into a cross over (dubbed an avoided quantum critical point). In turn, it is no longer obvious how robust the topological properties are in these materials. We will therefore discuss the effects disorder has on the robustness of Weyl Fermi arc surface states and the axial anomaly. We find that the Fermi arcs, in addition to having a finite lifetime from disorder broadening, hybridize with the non-perturbative bulk rare states, which unbinds them from the surface (i.e. they lose their purely surface spectral character). Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder. Lastly, we will discuss the robustness of the axial anomaly for a single Weyl cone in the presence of disorder. We will show that deep in the diffusive limit, when a band structure picture of dispersing (chiral) Landau levels no longer applies, the axial anomaly survives.