Hurricane on the Nanoscale
By Joshua Brown Article published March 1, 2006
He’s never seen one. Nobody has — yet. But Dennis Clougherty, professor of physics, believes he’s found something new to physics. He calls it’s a “Jahn-Teller soliton,” and his prediction about this strange form of matter, recently published in Physical Review Letters, may provide fresh understanding of the bizarre microscopic world of superconductors, buckyballs, and other so-called complex solids.
A soliton may be best explained by example. To find one you can see, travel to the Bay of Fundy in Nova Scotia. There, when the tide comes in, an odd wave called a tidal bore starts to move. This wall of water travels up low-lying rivers maintaining a steep slope — and flowing overtop the outgoing river water — for many miles at a constant speed without losing its form.
Another soliton can be found in Queensland, Australia. There, tubular clouds will develop, sometimes 600 miles long, that keep their shape even as they travel at 30 or more miles per hour. Other solitons include undersea waves that get pushed out from the Gulf Steam and travel thousands of miles through the ocean without dissipating. Tornadoes and hurricanes are examples too.
“These are all classical solitons,” says Clougherty, “but solitons also exist at the level of the electron.” And he believes he has found a new one at this smallest of scales, thousands of times smaller than a hair. His paper describes a “lump of excess electric charge in periodic motion,” maintained within the crystal structure of some ceramics and other complex materials. In other words, a kind of electric hurricane on the quantum scale.
“People are trying to understand why you get these kind of domains, these regions of inhomogeneity,” Clougherty says. “One possible explanation is that there is ‘dirt’ in these samples, maybe some kind of chemical contaminant.” But his models show something much more remarkable and elegant at work.
“What I am pointing out is that it is possible intrinsically, even in a pure sample, to have these kinds of inhomogeneities,” he says. “It turns out to be a non-linear system.”
A high degree of difficulty
A soliton at any size depends on a non-linear system of forces, meaning that when a substance is disturbed, whether water or wind or the electrons in superconducting yttrium barium copper oxide, the response is not in proportion to the disturbance. Instead, it continues unabated and nearly indefinitely. Crudely put, non-linear systems are more than the sum of their parts. This, of course, creates headaches for mathematicians and modelers, but non-linearity is at the root of effects like superconductivity, where electric charge moves without losing any juice.
If a soliton is strange, then the Jahn-Teller effect is one of the most difficult phenomena in physics. One way to describe it is as a geometrical distortion of the electron cloud around an atom in material where electrons can “choose” between several orbits of the same energy. Clougherty’s work shows that in some solids these off-kilter electrons, combined with a tweaking of the crystal “lattice” formed by the nuclei, keeps electric charge in splotches, despite the electrons’ natural inclination to separate from each other.
“This distortion of the lattice provides a mechanism for describing something called the structural phase transition,” Clougherty says. “There are lots of materials that have two different crystal structures, and you can go from one to the other by changing its environment, like the temperature.”
“There are several theories about the dynamics of how to go from one structure to another as the temperature is changed, and typically these kinds of structural phase transitions require everything to happen all at once,” he says. “But I think it’s possible that you might get a little domain with a warping of the structure inside this Jahn-Teller soliton, and as the temperature drops, the size of the soliton grows and eventually incorporates the entire sample.”
“So you’d have three phases,” he says, “the high temperature structure, the low temperature structure, and in between there would be a gas of solitons.”
Keith Johnson, a physicist at MIT and Clougherty’s advisor when he was a graduate student, notes that this paper is more than just gee-whiz theorizing. “Dennis’s work is very relevant to material science,” he says. It promises to provide a better framework for understanding some of the puzzling properties of electronic and optical materials that emerge from laboratories. Among these are superconductors like the ones used in MRI scanners, and fiber optic cables. Clougherty speculates that his work might have application in the current effort to build “quantum computers” that could store data at the scale of the electron.
Taking it to the lab
Now the trick is to test his mathematical model in the lab. “It’s never been observed in nature, but I’m hoping it will be,” he says. “I am hoping people will do careful experiments on these materials and find them.”
In his paper, Clougherty proposes several ways to see these Jahn-Teller solitons. “Inside the soliton you’d expect to see a distortion of the lattice,” he says. “That is something that can be seen with X-ray images.” Or, the splotches of excess electric charge in the soliton could be measured with an electron microscope.
“Publications in Physical Review Letters, especially for a theorist, are rare, and Dennis is absolutely a stunning top-drawer theoretical condensed matter physicist,” says Ken Golden, UVM professor of mathematics, who has worked with Clougherty. “I’d love to see UVM build up its physics program around Dennis.”
Clougherty will be presenting his theory this month at a meeting of the American Physical Society and has been invited to give a talk in August at the International Center for Theoretical Physics in Trieste.
“His work is very exciting,” MIT’s Keith Johnson says, “What he’s doing now is predicting something entirely new.”