Mar 15, 2007

herbertsmithite and string-net liquids

The most intriguing green crystal in the universe can't stop Superman, but it could be as debilitating as kryptonite to modern physics.
"Wen and Levin's theory is really beautiful stuff," says Michael Freedman, 1986 winner of the Fields medal, the highest prize in mathematics, and a quantum computing specialist at Microsoft Station Q at the University of California, Santa Barbara. "I admire their approach, which is to be suspicious of anything - electrons, photons, Maxwell's equations - that everyone else accepts as fundamental."

Other theories that try to explain the same phenomena abound, of course; Wen and Levin realise that the burden of proof is on them. It may not be far off. Their model predicts specific arrangements of atoms in the new state of matter, which they dub the "string-net liquid", and Joel Helton's group at MIT might have found it.

Helton was aware of Wen's work and decided to look for such materials. Trawling through geology journals, his team spotted a candidate - a dark green crystal that geologists stumbled across in the mountains of Chile in 1972. "The geologists named it after a mineralogist they really admired, Herbert Smith, labelled it and put it to one side," says team member Young Lee. "They didn't realise the potential herbertsmithite would have for physicists years later."

Herbertsmithite (pictured) is unusual because its electrons are arranged in a triangular lattice. Normally, electrons prefer to line up so that their spins are in the opposite direction to that of their immediate neighbours, but in a triangle this is impossible - there will always be neighbouring electrons spinning in the same direction. Wen and Levin's model shows that such a system would be a string-net liquid.

Although herbertsmithite exists in nature, the mineral contains impurities that disrupt any string-net signatures, says Lee. So Helton's team made a pure sample in the lab. "It was painstaking," says Lee. "It took us a full year to prepare it and another year to analyse it."
They were quite surprised with the results. You'll have to click through to find out why.

"Paradigm shift" is too twentieth century. Ready for a phase change?

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