Spin Control with a Topological Semimetal

Topological materials, from graphene to topological insulators, owe their remarkable properties to their two-dimensional surface states, which are protected from disorder and defects by topology and symmetry. A recently discovered class of materials, known as topological semimetals, often exhibit even richer and more robust topological effects. These materials include Dirac semimetals (DSMs) and Weyl semimetals (WSMs) [1, 2], which host electronic excitations behaving like Dirac and Weyl fermions, respectively. One of their intriguing properties is that the spins and momenta of their surface electrons are “locked.” Loosely speaking, this means that right-moving electrons are spin-up polarized, while left-moving ones are spin-down polarized—a behavior that can lead to exotic physics and may be harnessed in spintronic devices. However, it has been challenging to observe spin-polarized currents in these semimetals, mostly because currents are carried by both surface and bulk electrons, and spin-momentum locking isn’t as strong in the latter. Now, a team led by Zhi-Min Liao of Peking University has demonstrated a clever setup that reveals, with simple electrical measurements, spin-polarized transport in a DSM nanowire [3]. The researchers say that the setup can single out the contribution of surface electrons, eliminating that of bulk electrons. What’s more, the configuration allows the spin-polarized signal to be turned on and off with an applied voltage.