Massachusetts Institute of Technology

A theoretical physicist examines exotic particles lurking in new materials.

Axions are very light, very weakly interacting particles, whose existence was posited more than 30 years ago [1,2] in order to clean up our ‘standard model’ of particle physics [3]. They close an annoying loophole in Kobayashi and Maskawa’s Nobel-prize-winning explanation of why the microscopic laws of physics look so nearly the same when running backwards as forwards in time (time reversal symmetry).

Despite heroic efforts — and several false alarms — axions have not yet been detected, but they have become increasingly important. They have been warmly embraced in unified field theories and in string theory. And when we run the equations through Big-Bang cosmology, we find that axions should contribute much of the dark matter that astronomers have inferred to explain the Universe [5].

Now Shou-Cheng Zhang and his colleagues (X.-L. Qi et al. Phys. Rev. B78, 195424; 2008) inform us that, all along, axions have been lurking unrecognized on surfaces of bismuth-tin alloys and other materials. To be more precise: the equations that arise in axion physics [6,7] are the same as those that describe the electromagnetic behaviour of a recently discovered class of materials known, collectively, as topological insulators [8,9].
The axion field inside topological insulators is an emergent — and subtle — property of collections of electrons that is connected to their spin–orbit coupling.

These ‘quasi-axions’ don’t improve our standard model, but they do have the charming advantage of being accessible, possibly even useful. There are ideas to exploit their behaviour to make anyons [10], potential building blocks for quantum computation.

No short summary can do justice to the wealth of ideas synthesized in this paper. Powerful, beautiful mathematics is at play in reality.

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