Topological band crossings in hexagonal materials: Accordion states and star-shape Dirac lines
Topological semimetals exhibit protected band crossings near the Fermi energy, which give rise to a range of interesting phenomena: e.g., exotic surface states, anomalous magnetoelectric responses, and transverse Hall currents. These properties may open the possibility for new device applications, including spin-filter transistors or valleytronics applications. However, despite tremendous progress over the last few years, suitable materials for such applications remain scarce. In this talk, I present a classification of topological semimetals with hexagonal symmetry. Among them are novel Weyl semimetals with an accordion-like dispersion, as well as Dirac semimetals with star-shaped dispersions. I present a number of existing materials where these topological band crossings are realized near the Fermi energy. This includes γ-In2Se3 and ZrIrSn, which exhibit Weyl points and lines, respectively. The DFT band structures and surface states of these materials are discussed in detail.