Electronic structure of topological nodal semimetals

Topological phases in condensed matter physics have attracted intense worldwide interest in the past decade because these phases lie outside the framework of Landau’s spontaneous symmetry breaking paradigm. The early period of the field explored topological insulators (TIs). Three-dimensional (3D) TIs are distinct from the conventional band insulators in that they feature a bulk energy gap driven by spin-orbit coupling effects, and gapless surface states protected by time-reversal symmetry. In the last few years, the focus has shifted from insulators to topological semimetals (TSMs). Three types of TSMs have been identified, namely, Dirac, Weyl, and nodal-line semimetals. Dirac and Weyl semimetals are 3D analogs of graphene in which the bulk band structure disperses linearly from the nodal point in all three momentum directions, while the nodal-line semimetals display 1D nodal loops in the 3D Brillouin zone. In addition to the graphene-type (type-I) TSMs, the novel type-II TSMs where the band dispersion strongly violates Lorentz symmetry have also been studied extensively. In this talk, we will review a range of materials predictions including type-II Dirac semimetals [1], tunable Weyl semimetals [2], and nodal-line semimetals [3-5] by using first-principles calculations, and discuss various exotic phenomena observed experimentally in these compounds.

[1] T.-R. Chang et al, Phys. Rev. Lett. 119, 026404 (2017).
[2] T.-R. Chang et al, Nat. Commun. 7, 10639 (2016).
[3] G. Bian et al, Nat. Commun. 7, 10556 (2016).
[4] G. Bian et al, Phys. Rev. B 93,121113(R) (2016).
[5] T.-R. Chang et al, arXiv:1711.09167 (2017).