Topological semimetals

Topological semimetals give access to new quantum phenomena — for example, massless fermions have not been observed as elementary particles, yet they can be realized in the form of quasiparticles in these materials — and could allow the development of robust quantum devices.

Physicists have discovered a new topological phase of matter, the Weyl semimetal, whose surface features a non-closed Fermi surface whereas the low-energy quasiparticles in the bulk emerge as Weyl fermions. A brief review of these developments and perspectives on the next steps forward are presented.

Topological semimetals and metals have emerged as a new frontier in the field of quantum materials. Novel macroscopic quantum phenomena they exhibit are not only of fundamental interest, but may hold some potential for technological applications.

The half-Heusler GdPtBi is found to show transport and calorimetric signatures of the existence of Weyl fermions under the application of a magnetic field. The half-Heusler alloys form a big family of tunable compounds that may substantially enlarge the number of Weyl semimetals known.

MoTe₂ is reported to host type II topological Weyl semimetal states. Two sets of Weyl points exist at different energies above the Fermi energy. Fermi arcs that form closed loops and are unique to type II Weyl semimetals are also found.

The half-Heusler GdPtBi is reported to exhibit negative longitudinal magnetoresistance. This is attributed to the chiral anomaly due to the formation of Weyl nodes with an applied magnetic field. The anomaly is also found to suppress the thermopower.

Studying three Weyl semimetals of the same family — TaAs, TaP and NbP — reveals how the properties of Weyl points and Fermi arcs depend on the spin–orbit coupling and on surface conditions.

Band structures with Fermi arcs characteristic of Weyl semimetals are observed on NbP and TaP. By studying NbP, TaP and TaAs, the evolution of the Fermi surface with the spin–orbit coupling is reported.

A state of matter known as a three-dimensional Dirac semimetal has latterly garnered significant theoretical and experimental attention. Using angle-resolved photoelectron spectroscopy, it is shown that Cd₃As₂ is an experimental realization of a three-dimensional Dirac semimetal that is stable at ambient conditions.

Three-dimensional Dirac semimetals such as Cd₃As₂ are attracting attention because their electronic structure can be considered to be the three-dimensional analogue of graphene’s. Low-temperature scanning tunnelling measurements of the 112 cleavage plane of Cd₃As₂ now reveal its electronic structure down to atomic length scales, as well as its Landau spectrum and quasiparticle interference pattern.

Three-dimensional analogues of graphene have recently been synthesized. The transport properties of such a Dirac semimetal, Cd₃As₂, have been studied, revealing an unexpected mechanism that suppresses backscattering dramatically.