Type-II Weyl Semimetals

TL;DR

This paper predicts a new type of Weyl fermion in condensed matter, emerging at the boundary of electron and hole pockets in type-II Weyl semimetals, exemplified by WTe₂, with unique physical properties.

Contribution

It introduces a novel type of Weyl fermion that breaks Lorentz symmetry and identifies WTe₂ as a material hosting this particle as a low-energy excitation.

Findings

Discovery of a new Weyl fermion type in condensed matter.

Identification of WTe₂ as a candidate material for type-II Weyl semimetals.

Unique Fermi surface with finite density of states at the Weyl node.

Abstract

Fermions in nature come in several types: Dirac, Majorana and Weyl are theoretically thought to form a complete list. Even though Majorana and Weyl fermions have for decades remained experimentally elusive, condensed matter has recently emerged as fertile ground for their discovery as low energy excitations of realistic materials. Here we show the existence of yet another particle - a new type of Weyl fermion - that emerges at the boundary between electron and hole pockets in a new type of Weyl semimetal phase of matter. This fermion was missed by Weyl in 1929 due to its breaking of the stringent Lorentz symmetry of high-energy physics. Lorentz invariance however is not present in condensed matter physics, and we predict that an established material, WTe$_2$, is an example of this novel type of topological semimetal hosting the new particle as a low energy excitation around a type-2 Weyl node. This node, although still a protected crossing, has an open, finite-density of states Fermi surface, likely resulting in a plethora physical properties very different from those of standard point-like Fermi surface Weyl points.