Phys. Rev. B 92, 201102(R) (2015)

Despite being the first of now many classes of exciting materials whose properties are enriched by the topology of the electronic states, topological insulators still pose a number of challenges. Eric de Vries and colleagues have given us a hint of the sort of problem we're facing.

Ideally insulating in the bulk, but with topologically protected conducting surface states, topological insulators have a number of properties that could be exploited in devices. For the spintronics community, the direct coupling between the spin and momentum of the charge carriers in the surface states offers the prospect of injecting and detecting spin currents without the need for ferromagnetic materials.

But de Vries et al. showed that simply measuring the signals due to spins in the surface states of topological insulators is not so easy. With devices capable of measuring the spin signals due to spin–momentum locking in different geometries, they were able to see clear evidence for a spin polarization of the surface state. The problem is that they can see these signals when spin–momentum locking was clearly not the cause.