title: Type-II Weyl semimetals.
abstract: Fermions--elementary particles such as electrons--are classified as Dirac, Majorana or Weyl. Majorana and Weyl fermions had not been observed experimentally until the recent discovery of condensed matter systems such as topological superconductors and semimetals, in which they arise as low-energy excitations. Here we propose the existence of a previously overlooked type of Weyl fermion that emerges at the boundary between electron and hole pockets in a new phase of matter. This particle was missed by Weyl because it breaks the stringent Lorentz symmetry in high-energy physics. Lorentz invariance, however, is not present in condensed matter physics, and by generalizing the Dirac equation, we find the new type of Weyl fermion. In particular, whereas Weyl semimetals--materials hosting Weyl fermions--were previously thought to have standard Weyl points with a point-like Fermi surface (which we refer to as type-I), we discover a type-II Weyl point, which is still a protected crossing, but appears at the contact of electron and hole pockets in type-II Weyl semimetals. We predict that WTe2 is an example of a topological semimetal hosting the new particle as a low-energy excitation around such a type-II Weyl point. The existence of type-II Weyl points in WTe2 means that many of its physical properties are very different to those of standard Weyl semimetals with point-like Fermi surfaces.
authors.affiliation: Theoretische Physik and Station Q Zurich, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Alexey A
authors.initials: AA
authors.lastname: Soluyanov
authors.affiliation: Theoretische Physik and Station Q Zurich, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Dominik
authors.initials: D
authors.lastname: Gresch
authors.affiliation: Department of Physics, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Zhijun
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Theoretische Physik and Station Q Zurich, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: QuanSheng
authors.initials: Q
authors.lastname: Wu
authors.affiliation: Theoretische Physik and Station Q Zurich, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Troyer
authors.affiliation: Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Xi
authors.initials: X
authors.lastname: Dai
authors.affiliation: Department of Physics, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: B Andrei
authors.initials: BA
authors.lastname: Bernevig