World of Weyl Craft
Over the last year, much excitement has surrounded Weyl semimetals. These materials have an asymmetric crystal structure that results in never-before-seen collective excitations called Weyl fermions. Hints have also emerged that transition metal dichalcogenides, such as molybdenum ditelluride ( ), represent a distinct category of Weyl semimetals (type-II), characterized by bizarre symmetry properties. Anna Tamai from the University of Geneva, Switzerland, and colleagues performed a careful assessment of and argue that it is indeed a strong candidate for a type-II Weyl semimetal.
Weyl semimetals have a complex electronic band structure, in which two bands meet at points. In a type-I Weyl semimetal (see 8 September 2015 Viewpoint), these so-called Weyl points are connected by arc-shaped features, known as Fermi arcs, which can be observed in data obtained with angle-resolved photoemission spectroscopy (ARPES). A type-II Weyl semimetal would also exhibit Fermi arcs, but the endpoints would not correspond to the Weyl points—making them harder to identify. This difference arises because the type-II band structure is predicted to have a large tilt, resulting in Weyl fermions that violate Lorentz symmetry. This violation would produce exotic properties such as the material acting as a conductor for electrons moving in certain directions, while being an insulator in others, depending on the orientation of an applied magnetic field.
Several groups claimed to have observed a type-II Weyl semimetal. However, Tamai et al. argue that candidate materials may exhibit arcs that are actually “false positives.” Bearing this in mind, the authors identified several arc-like features in their ARPES data for and then compared them to detailed electronic-structure calculations. They showed that some of these arcs can be explained without Weyl points, but others are only reproduced in scenarios with at least eight Weyl points, consistent with being a type-II Weyl semimetal.
This research is published in Physical Review X.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.