Researchers find what may be a new state of matter

[Ace Pace]

Ars

Think way back to elementary or primary school, somewhere around third-grade physical science, when you first learned about the various states of matter. At the time you were undoubtedly told that there were three states of matter: solid, liquid, and gas. Solid is where the atoms are tightly packed into some arrangement and vibrate in place; liquids have more freedom of motion and vibration, allowing them to take on any bulk shape; gas molecules had near complete freedom of motion and rarely saw another molecule. Perhaps later you learned about plasma (molecules where the electrons have been completely stripped from the nucleus) as a fourth state, but for most people their education regarding states of matter ends around there. New work by a pair of theoretical physicists studying an odd quantum mechanical effect may reveal a new state of matter, and if their model is shown to be accurate, it will completely change how we view the universe.

Quasi-particles

About ten years ago the Nobel prize in physics went to a team for their discovery of what they termed quasi-particles. These were particles that seemed to have a fractional charge, something that should not be possible according to physics. They were seen in an experiment where electrons moved across an interface between two semiconducting materials, and they behaved as though they did not have an integer charge on them. This phenomena, termed the fractional quantum Hall effect (FQHE), suggested that electrons are NOT a fundamental particle in our universe. Were this to be true, much of physics would need to be rethought. However, it was found that this observed fractional charge was due to the positions of a group of electrons near one another. This positioning gave the illusion of a fractional charge. The charge on the electron was correct and physics could rest easy.

This idea of quasi-particles clearly intrigued people; Xiao-Gang Wen at MIT and Micheal Levin at Harvard University began to look at this in further detail. They felt that the electrons in this arrangement could represent a new state of matter. According to Wen, when you look at the electrons in the FQHE system, they appear in a random position similar to a liquid. However, the electrons move in a well-correlated manner, as if they were all entangled. Wen speculated that this potentially new state of matter is one where quantum entanglement is a basic property.

String-nets



Using this idea, Wen and Levin postulated something incredible: what if electrons were not fundamental particles, but merely the ends of long strings of other "true" fundamental particles? With this idea, the pair developed a new model for matter where it was made up of these strings woven together to form what they dubbed "string-nets." Developing and running computer simulations of this model showed that it gave rise to both conventional particles as well as the quasi-particles, those which carried a fractional charge that began this whole adventure. Then they discovered something completely unexpected.

As the "string-net" vibrated, it produced waves as any vibrating net would, but the pair soon realized that the waves being emitted by the "string-net" behaved exactly as they would according to Maxwell's equations—the set of equations that describe how electromagnetic radiation behaves! As Wen himself says "A hundred and fifty years after Maxwell wrote them down, here they emerged by accident." While a huge success, the model didn't stop there: Wen and Levin showed that the model could give rise to quarks—which make up protons and neutrons—as well as the force particles: gluons and W and Z bosons! From here, the researchers made a huge leap to suggest that this model may be capable of explaining the structure of the universe. If it can explain how both light and matter can be formed from the vacuum of space, perhaps they are onto something big. (Prof. Wen's lecture on this topic is available online.)

At this point any good scientist will be reading with a grain of salt; it isn't every day that someone suggests that what physics considers as the basic building blocks of the universe are called into question. Many did. According to Michael Freedman, winner of the Fields medal in 1986 and quantum computing expert at Microsoft, "Wen and Levin's theory is really beautiful stuff. I admire their approach, which is to be suspicious of anything–electrons, photons, Maxwell's equations–that everyone else accepts as fundamental."

A very unusual mineral

While this could have been relegated to wherever science puts the many other theories that have tried to explain the universe, this idea received a big boost from an unlikely source. In 1972 geologists unearthed a new type of mineral in Chile; this mineral, now known as herbertsmithite, has a very unusual property: its electrons are arranged in a triangular lattice. This is atypical since electrons usually line up in pairs so they can have the opposite spin as their nearest neighbor. In a triangle, two neighbors will be forced to have the same spin. According to Wen and Levin, this ternary electron system would be a string-net liquid, their new state of matter.

Recently another group at MIT who were aware of Wen and Levin's theory started trying to synthesize this material, herbertsmithite, without defects so it could be studied in detail. After achieving the goal of growing the mineral, they began to characterize it and found that it was unlike many other minerals on Earth. Most minerals exhibit what is called magnetic ordering—where the electron spins in a lattice line up. In the synthesized material no such ordering was found no matter how cold they made the crystal. Even at a fraction of a degree Kelvin, no magnetic order appeared. The team also measured the heat capacity of the material; often this will change below a certain temperature due to a structural change in the material. Again, no such change was found even down to incredibly low temperatures. According to Y. S. Lee, the MIT professor who oversees the lab where the herbertsmithite was synthesized, "We could have created something in the lab that nobody has seen before."

Further studies into the details of the possible entanglement properties of this material are being undertaken now. The team, led by Prof. Lee, is firing neutrons into the material to see if there is any long-range correlation in the motion of the electrons, indicating some degree of entanglement. While these are both radical concepts—one challenges the very fundamentals of physics while the other suggesting a type of material never before studied—the experiments and theories are in the early stages. Even if they do not pan out, it is worth noting that there are many other groups around the world looking for new states of matter every day. Michael Freedman rightly notes that often people think new states of matter only occur in the high temperatures and energies of particle accelerators which, as he puts, "are just recreating conditions after the big bang and repeating experiments that are old hat for the universe." New materials and theories are being discovered all the time.

[Josh]

Herbertsmithite, eh? I wonder if that old friend of mine ever got around to naming the new species of fungus she found after me...

[Lord Iames Osari]

[Spock]Fascinating.[/Spock]