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Big step to high-temperature superconductors

Posted: 16 Aug 2005 ?? ?Print Version ?Bookmark and Share

Keywords:ultracold superfluid gas? fermi gas? superconductivity? superfluidity? lithium ion?

Rapid progress with ultracold superfluid gases in the lab over the past year has physicists confident that a full explanation of high-critical-temperature superconductivity is just around the corner.

A year ago, three experimental-physics groups reported the breakthrough creation of an electron-like gas in which all the atoms shared the same quantum state. The same thing happens when electrons in a metal make the transition from normal to superconducting flow, where electrical resistance totally disappears. The difficulty of the gas experiments, however, made it impossible to determine whether the resulting "Fermi gas" was actually a superfluid, which is able to flow without resistance.

Now a group of researchers at the Massachusetts Institute of Technology's Center for Ultracold Atoms has performed a definitive experiment that clearly reveals the superfluid nature of these Fermi gases. The experimental system consisted of only 3 million lithium ions cooled to 50 nanoKelvin using laser beams to slow their movement. Since the atoms are much heavier and their interaction distances much larger than electrons in a conductor, however, a corresponding electron system would be superfluid above room temperature.

"Superconductivity is superfluidity for charged particles, like electrons in a metal. Superfluidity in a gas of atoms has so far only been observed in a Bose-Einstein condensate," said Christian Schunck, a member of the team that performed the experiment. Schunck was referring to the original experiments, performed independently at MIT and the University of Colorado, Boulder, in 1995 in which ultracold atoms were observed to condense into a single quantum state-the Bose-Einstein condensation predicted, but never actually observed, by Albert Einstein.

"What we have observed now is different: It is high-temperature superfluidity in a strongly interacting gas of fermionic lithium atoms. This is the major achievement of our work," MIT's Schunck said.

The basic physical fact behind Bose-Einstein condensation is the ability of bosonsparticles with whole-number quantum "spin"to share the same quantum state when their velocity slows sufficiently, representing a temperature very close to absolute zero.

Electrons, by contrast, belong to a complementary class of particles called fermions and do not have integer spin but, rather, half-integer spin. The fermions can enter a superfluid state at low temperatures by pairing up so that the spin of each pair becomes an integer; when that happens, the pairs represent bosonic particles and are able to share the same quantum state and flow without resistance.

A continuing mystery has been how electrons in high-critical-temperature ceramic superconductors achieve resistance-free flow. Thus, the recent MIT experiment has the physics community asking whether the superfluid fermion gas might represent high-critical-temperature superconductivity.

"Electrons in a superconductor are also fermions, and they also form pairs. That is why the observation of high-temperature superfluidity in our gas and high-temperature superconductivity are so intimately related," said Schunck. "The macroscopic physics of this pairing is not well-understood for high-temperature superconductors. By studying high-temperature superfluidity in our system, we hope to gain a better understanding of this physics."

A useful aspect of the fermionic gas system is the ability to smoothly vary the strength of pairing between the lithium atoms. The pairing is created by an oscillating magnetic field and can be adjusted by varying the field strength. Loosely coupled atoms model low-temperature superconductivity. As the coupling becomes tighter, the corresponding "temperature" of the group of electrons modeled by the gas increases.

Thus, at lower field strengths, the lithium atoms are strongly paired into particles that look very much like bosons, creating an accurate model of a Bose-Einstein condensate. This also happens to be a high-temperature superfluid, suggesting that further work with such systems could crack the mystery of high-Tc superconductivity.

Since the first observation of fermionic gases last year, the MIT group has been busy refining the basic techniques. The researchers first had to create an accurate spherical shape for the gas using lasers to confine the atoms in a vacuum. Next, they needed to find a way to impart angular momentum to the sphere. Using a special laser beam to "stir" the clump of atoms proved to be a practical method.

A critical property of superfluids is the inability to rotate as normal matter does. In a rotating system, the individual atoms follow different paths with different velocities, which would be impossible if they all shared the same quantum state. Superfluids absorb rotational momentum by forming one-dimensional singularities. The line-like vortices repel one another and form a distinct regular pattern.

Finally, the group had to find some way to image the pattern of singularities. It accomplished that by suddenly allowing the gas to expand while rapidly increasing the field. The two physical processes allow the cloud to enlarge while maintaining its configuration. The distinct signature of a regular lattice of vortices was observed, confirming the superfluid state of the clump of atoms.

The first Bose-Einstein condensate, comprising about 2.000 rubidium atoms, was created in 1995 at the JILA institute, a joint facility operated by the University of Colorado-Boulder and the National Institute of Standards and Technology. A few months later, Wolfgang Ketterle, who heads MIT's Center for Cold Atoms, independently created a Bose-Einstein condensate. Eric Cornell and Carl Wieman at JILA and Ketterle jointly received the Nobel prize for Physics in 1997 for their achievements.

Last year, three groupsKetterle's at MIT, Rudolf Grimm's at the University of Innsbruck (Austria) and Deborah Jin's at JILAdemonstrated the first condensates created by coupled fermionic atoms.

The refinement of experimental techniques for creating superfluid atomic gases could lead to breakthrough models of high-Tc superconduction. An intriguing possibility would be the use of interacting light fields to create the physical equivalent of the atomic lattices that electrons see inside a conductor.

If researchers could create close models of such systems, they might finally get a grip on a vital electronic phenomenon with a wide range of practical applications.

- Chappell Brown

EE Times




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