Published online 16 March 2010 | Nature | doi:10.1038/news.2010.128


Are the Universe's secrets hiding on a chip?

Topological insulator could help to test quantum field theory.

Conceptual computer artwork of a sphere on a circuit board within a beam of light, representing how data may be controlled and stored in a quantum computer.Topological insulators could be the next testing ground for particle physics.M. KULYK / SCIENCE PHOTO LIBRARY

An obscure class of materials could be used to simulate a slew of exotic particles predicted by physicists but never seen.

Preliminary results presented on 14 March on the eve of the American Physical Society's meeting in Portland, Oregon, suggest that a large enough chunk of a 'topological insulator' has been made to test some of the odd predictions of quantum field theory — a version of quantum mechanics that is commonly used in particle physics. The theory predicts the existence of a number of unusual particles, which if reproduced in the material could prove useful for future applications such as code-cracking quantum computers, or in spintronics — electronics that relies on particles' spin as well as their charge.

Now Laurens Molenkamp, a physicist at the University of Würzburg in Germany, believes that he has created a mercury telluride (HgTe) topological insulator thick enough to put the theory through its paces.

Topological insulators are materials that conduct electrons on the outside but act as insulators on the inside. The origin of that seemingly mundane property lies in the way that electrons move through the material. Electrons carry a quantum mechanical 'spin' that points either 'up' or 'down'. Spin is normally independent of an electron's motion, but inside topological insulators, electrons' spins are strongly related to how they move.

'Multiverse' on a chip

That relationship between spin and motion makes the insulators a good medium in which to model some formulations of quantum field theory, says Shoucheng Zhang, a theoretical physicist at Stanford University in California.

Quantum field theory has been extraordinarily successful in describing the Universe, but some of its predictions have proved difficult to verify. Some formulations suggest the existence of axions — weakly interacting particles proposed to account for unseen 'dark matter', which could make up almost a quarter of the Universe's mass. The theory also allows for the existence of magnetic monopoles, points of individual north and south that have never been seen in nature.

"We live in one kind of universe, but inside these solids you can create these unusual universes," says Ali Yazdani, a physicist at Princeton University in New Jersey. "That's cool."

“We live in one kind of universe, but inside these solids you can create these unusual universes.”

The particles wouldn't be the same as those predicted by quantum field theory — for instance, a study by Zhang and his colleagues shows that axions could be simulated as magnetic fluctuations inside a topological insulator1. But the analogy could guide scientists on where to look for the particle's real equivalents in the Universe. Shining polarized light through the insulator could reveal telltale signs of axions. If axions really do exist, then the same signature might also appear in the cosmic microwave background radiation, the primordial radiation left over from the Big Bang.

Some of the proposed exotic particles could also have practical uses. One class, known as Majorana fermions, predicted to be very stable, could be used in quantum computers to store data.

Funky things

The HgTe used by Molenkamp is a well known topological insulator, but so far the topological insulating behaviour has been seen only along the edges of razor thin slivers of the material. In preliminary results presented at a tutorial ahead of the meeting, Molenkamp revealed evidence that electrons on the surface of his three-dimensional sample were behaving as though they were in a topological insulator. "If this is all working, we can experimentally check quantum field theory," he says.


If HgTe lives up to his expectations, Molenkamp says he may soon begin the search for the "funky things" predicted to reside inside it.

Yazdani, who works with an alternative class of materials based on bismuth, says that if Molenkamp has achieved the results he describes, this would be a significant step forwards for the field. But, he adds, "I haven't seen his data so I can't say how convincing it is."

Zhang says that the results are exciting. However, he acknowledges that although axions and monopoles might live in a topological insulator, that doesn't mean they'll exist in the real world. "It doesn't mean that we will see it in the Universe," he says. "But at least it tells us these equations are not crazy." 

  • References

    1. Li, R., Wang, J., Qi, X.-L. & Zhang, S.-C. Nature Phys. doi:10.1038/nphys1534 (2010).
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