Hints of Universal Behavior Seen in Exotic Three-Atom States
|12:56:14 AM, Tuesday, September 27, 2011|
"A novel type of inter-particle binding predicted in 1970 and observed for the first time in 2006, is forming the basis for an intriguing kind of ultracold quantum chemistry. Chilled to nano-kelvin temperatures, cesium atoms -- three at a time -- come together to form a bound state hundreds or even thousands of times larger than individual atoms. Unlike the case of ordinary atoms, wherein electrons are bound to a nucleus in a spectrum of energy levels on the order of an electron volt (that is, it would typically take an eV of energy to free the electron), the cesium triplets feature energy levels that are measured in trillionths of an electron volt (peV). Stranger still, a new experiment observing four such cesium states reports that the states' sizes are roughly the same. This has taken theorists by complete surprise.
In the seventeenth century Isaac Newton derived the classical force laws used to calculate the force between two objects. Calculating the behavior of three-body groupings such as the Moon/Earth/Sun system was much harder; indeed Newton never succeeded. Nowadays such problems can be studied with powerful computers, but only numerical simulations are possible, and not exact, analytical solutions.
In 1970, however, Russian physicist Vitaly Efimov predicted that under some special conditions, three bodies, such as atoms at ultralow temperatures, could be made to enter into stable states whose behavior could be calculated with remarkable ease. Then in 2006 exactly such states were actually observed by scientists at the University of Innsbruck. Now, these researchers have extended their work and demonstrated that the "three-body parameter," used to describe how the three participating bodies interact, varies in a consistent way regardless of the atomic species used.
Paul Julienne, a scientist at the Joint Quantum Institute (JQI), operated by the University of Maryland and the National Institute of Standards and Technology (NIST), contributed theoretical help to the Innsbruck scientists conducting the experiment, a team led by Rudolf Grimm. "None of the experts in three-body physics had expected this kind of universal behavior to show up in these 3-atom systems," Julienne said. "This behavior came as a big surprise." And the universality, in turn, might suggest the existence of some new kind of ultracold chemistry at work.
Efimov's 1970 work met with much skepticism, especially since his prediction specified that three particles could form stable partnerships even though none of the two-particle matchups were stable. That is, 3 particles could accomplish what 2 particles could not. This novel arrangement has been compared to the "Borromean Rings," a set of three rings used on heraldic symbol for the Borromeo family during the Italian Renaissance. The three rings hold together unless any one of the rings is removed..."