by Mark Saxton
Recent developments in the studies of electron spin properties in materials have resulted in the discovery of a new type of material showing very unusual electrical conductivity behaviour. Known as topological insulators, these materials are conventional insulating materials (i.e. they are very poor conductors of electricity), except that, in very thin samples, quantum effects allow the movement of electrons along the surface of the material.
As a result of the above, these materials are conducting only on their surface and not in the bulk of the substance. Both the motion of the electrons along the surface and their spin characteristics can be manipulated and this could result in important applications in high-speed computing and also in the simulation of high energy particles predicted to exist by nuclear physicists, but so far unobserved.
The basic band structure of a topological insulator is that of a normal insulator, with the Fermi level lying between the conduction and valence bands (see diagram). However, at the surface of the material there are electron states with energies falling within the bulk energy gap; these are free to move and thus allow conduction.
The mechanism behind this phenomenon is very complex, but is related to the quantum spin Hall effect: Quantum mechanics shows that the electrons are spinning. Normally the direction of spin is constantly changing because of external effects such as collisions, but in topological insulators the quantum spin of surface electrons is aligned with their direction of motion.
Also, because the electrons exist in protected ‘edge states’ on the surface of the material, their quantum spin is not subjected to the usual changes of direction. All of this means that there is potential for these materials to be used in ‘spintronics’ devices, where the electron spin orientation is used to carry and encode information and possibly produce much faster, more efficient electronic devices than at present.
There are currently over 150 articles on the Inspec database containing the phrase ‘topological insulators’, mainly from 2010, and Inspec has recognised the current levels of interest in this area by introducing a new Thesaurus term for 2011:
There are also a number of existing thesaurus terms and classification codes relevant to the science of topological insulators: