Physicists in the US have shown that electrons in topological surface states keep their high conductivity despite the presence of surface defects. The experiment confirms a key prediction about the behaviour of electrons on the surface of a “topological insulator” – a newly discovered state of matter that promises a wealth of new physics and that could have practical applications in electronics and quantum computing.
Topological insulators are of great interest in condensed-matter physics because of their unusual conduction properties. Although electricity does not flow easily through the bulk of these materials – hence their name – current does flow well on their surface. Topological insulators could therefore be used as ultrathin conductors for electronic devices, and may even harbour new types of quasiparticle that are insensitive to the environmental noise that plagues quantum computers.
The strange properties of topological insulators arise because the shape – or topology – of the electron energy bands makes it impossible for a surface electron to backscatter. If such an electron is on a terraced crystalline surface with successive atomic steps, theory predicts that the electron will not be scattered if it travels perpendicular to the steps. This is unlike electrons on the surfaces of normal metals, which are strongly reflected at steps.
On the terraces
Ali Yazdani and colleagues at Princeton University set out to test this prediction by studying electrons on the surface of a single crystal of antimony. Antimony is a semi-metal and therefore not strictly a true topological insulator, but it does have topological surface states.
The team began by placing an antimony crystal in an ultrahigh vacuum chamber, where it was cleaved to expose a clean terraced surface. The sample was then cooled to 4 K and its surface was analysed using a scanning tunnelling microscope (STM).
The STM measures the density of electron energy states on the surface. These electrons tend to be confined to individual terraces and their energy states are simply the solution to that classic problem of undergraduate quantum physics – the particle in a box.
Half and half
However, because the electrons exist in topological states they can “leak” out of the terraces if they are moving perpendicular to a step. This leakage widens particle-in-a-box energy states, which can be seen in the STM images. By measuring their width, Yazdani and team worked out that electrons in the topological surface states were transmitted across a step edge about 50% of the time, and reflected the other 50%. By contrast, when similar experiments are done using normal metals such as copper there is effectively zero transmission across step edges.
According to Yazdani, the transmission is not 100% because antimony’s electron states are more complicated than topological surface states on other materials. The transport of topological surface electrons is also dependent upon the direction of their spin – and in antimony the band structure and spin properties are such that the surface electrons can both transmit through and reflect from defects. In other materials, reflection can be completely forbidden.
Important feature
Indeed, the ability to measure both transmission and reflection is an important feature of the experiment. While the STM technique works very well on antimony, Yazdani points out that it is not appropriate for studying the surfaces of some other topological insulators where there is little reflection
The work is reported in Nature.
