By Hamish Johnston in Portland, Oregon
Zhang (left) next to Molenkamp
There was a slight panic here in the press room over lunch when we all realized that we will soon be writing about topological insulators. We weren’t exactly sure what they were – but it’s becoming clear that topological insulators are the hot topic here at the APS March Meeting.
Fortunately we received a good introduction by some of the leading lights in the field, including Shou-Cheng Zhang of Stanford University. Zhang described topological insulators as “a new state of matter that has been predicted and discovered”. The prediction – by Zhang, I believe – occurred in 2006 and the first material was made a year later by Laurens Molenkamp at the University of Würzburg, who was also at the press conference.
Topological insulators are actually pretty good conductors (more on that later) and could lead to smaller integrated circuits that run faster and cooler. There is even the suggestion that axions and Majorana fermions could be lurking in these materials.
A simple description of a topological insulator is a material that is an insulator in the bulk, but a very good conductor on the surface.
Why? Well, it seems to have something to do with the quantum spin Hall effect – the accumulation of electrons with opposite spins on opposite sides of a conductor.
Let’s say a spin-up electron is flowing along the surface and scatters off an impurity.
The scattering process involves orbital angular momentum – and thanks to spin-orbit coupling, the spin of the electron is also rotated during the scattering.
Here’s the tricky bit that I didn’t quite understand. If the electron is scattered backwards the spin rotation introduces a phase shift of –1. If you think of this scattering as wave diffraction, destructive interference means that the electron can’t propagate in the opposite direction.
No backscattering means that the resistance of the material is very low, which is very useful if you are trying to make very tiny electronic circuits.
Sounds reasonable, but there are a few things I don’t understand. For one thing, this explanation seems to hinge on the electron only being able to scatter forwards or backwards – but not off to the side.
I’d better start reading-up on topological insulators.