8 August 2012
Three condensed-matter physicists, who have advanced our understanding of a strange type of material known as a “topological insulator”, have won this year’s Dirac medal from the International Centre for Theoretical Physics in Trieste, Italy. Duncan Haldane of Princeton University, Charles Kane of the University of Pennsylvania and Shoucheng Zhang of Stanford University, all in the US, have scooped the $5000 prize, which is named after the British Nobel-prize-winning theorist Paul Dirac. First awarded in 1985, the prize is given each year on 8 August – the day on which Dirac was born in 1902.
At the surface
Topological insulators are currently one of the hottest topics in condensed-matter physics. Insulators on the inside, they manage to conduct electricity on their surface thanks to special surface electronic states that are “topologically protected”, which means that – unlike ordinary surface states – they cannot be destroyed by impurities or imperfections. Moreover, the conducting electrons arrange themselves into spin-up electrons travelling in one direction and spin-down electrons travelling in the other. Such a “spin current” could be useful for anyone wishing to build a practical “spintronic” device that exploits the spin, rather than the charge, of the electrons.
These insulators have an unusual history because – unlike almost every other exotic phase of matter – they were characterized theoretically before being discovered experimentally. Kane was among those who were involved in that early work, which was based on the band theory of solids – the standard quantum-mechanical framework for understanding the electronic properties of materials. The topological insulator states in 2D and 3D materials were predicted theoretically in 2005 and 2007, before being experimentally discovered in 2007.
Given that the 3D topological insulators are fairly standard bulk semiconductors and their topological characteristics can survive to high temperatures, their novel properties could lead to some exciting applications. But as well as constituting a new phase of quantum matter that should keep physicists busy for some time, topological insulators have also aroused interest because they have been shown to harbour quasiparticles resembling “Majorana fermions” – particles that are also their own antiparticle.
But it is the potential applications of topological insulators that drives much of the current interest in these materials. “When developing nanosize electronics and pursuing coveted goals such as quantum computers, the availability of conducting channels that do not spoil and will work no matter what is something that seems quite important,” says ICTP condensed-matter physicist Erio Tosatti. The Dirac medal is awarded annually but anyone who has already won a Nobel prize, Fields medal or Wolf Foundation prize is excluded from the award.