A news piece for Materials Today – originally appeared here: April 2013, Vol 16, No. 4
A team of scientists from Berkeley have shown that the spin polarization of photoelectrons produced by topological insulators can be controlled using a laser.
Topological insulators (TIs) do not act like any other material on Earth and it is their electronic properties that make them interesting. On their surface, TIs behave like metals, but in their bulk, they are insulating. And the surface electrons are always spin polarized, meaning that electron’s spin is dependent on its momentum, and always perpendicular to the electron’s direction of travel. TIs also retain these properties at room temperature, making them a hot topic in materials science.
A Nature Physics paper from a team of Berkeley scientists has reported on an unexpected property of these materials – they showed that when topological insulators are illuminated with a UV-laser,the spin polarization of the photoelectrons (produced via the photoelectric effect) can be completely controlled, simply by varying the polarization of the light.
The team used a technique called ARPES (Angle-Resolved PhotoEmission Spectroscopy) to study Bismuth Selenide, a well-known TI. ARPES probes the quantum-mechanical behaviours of materials, by mapping electronic band structure. In a metal, the conduction and valence bands overlap, allowing for free motion of electrons. In an insulator, there is a gap between the bands, which limits electron transfer. But in TIs, the bands appear as cones which meet at a single point (Dirac point) resulting in the complex electronic properties we see.
The real breakthrough came in the development of Berkeley’s spin time-of-flight analyser, which measured electron spin by analysing the scatter of photoelectrons from a magnetic surface. Their first experiments used a parallel-polarized laser, and results showed that the emitted photoelectrons were spin-polarized – consistent with accepted theory. The team then reversed the beam’s polarization, and found that this reversal also reversed the spin-polarization direction of the emitted photoelectrons; a result which ran counter to all accepted knowledge on photoemission in TIs. They went on to show the same effect was present when circularly-polarized light was used.
This discovery has shown that it is possible to optically control electronic properties, which may help to realise the full potential of spintronics (electronics that exploit can spin as well as charge), long predicted to herald in the quantum computing revolution. This work may also have a more immediate application in electron based imaging technologies – an electron beam with controllable spin polarization will offer a new route into the heart of materials.
Nature Physics (2013) doi:10.1038/nphys2572