15 March 2006

The Strange Matter Of Superfluidity

by Kate Melville

In his most recent experiments into superconductivity and superfluidity, physicist Randall Hulet of Rice University looks to have recreated the same quantum conditions that may occur at the heart of quark stars; the stars that are believed to be comprised of strange matter. Hulet's observations fly in the face of conventional theories, because he has shown that there does not need to be an equal number of spin-up and spin-down particles for superfluidity to take place.

In previous experiments, appearing in the journal Science, Hulet reported that an unbalanced superfluid comprised of fermions (all elementary particles are either fermions or bosons) could exist. "Conventional theory tells us superconductivity or superfluidity occurs only in the presence of an equal number of spin-up and spin-down particles," said Hulet. But his experiments show that lone particles do exist, and that an ideal quantum coupling harmony is not required.

For the past half century there has been widespread speculation among physicists about the nature of such a phenomenon, but none of them, it seems, expected to observe a bunch of married fermion pairs surrounded by lonely single fermions. "Because of the pristine and controlled nature of ultracold atoms, we're able to offer definitive evidence of what happens with mismatched numbers of spin-up and spin-down particles," said Hulet.

Quantum physics is a very orderly, rule based world, where every miniscule bit of matter has what is called "spin". Like a passport, spin is a fixed identifier of a particle that determines what other particles it can mix with in quantum space. As a result, fermions don't really associate with other quantum particles very well, but physicists use them because fermion pairings make superconductivity and superfluidity possible. Superconducting and superfluid phases are similar, except that the particles in superconductivity carry an electrical charge, while superfluidity takes place in electrically neutral particles. But both superconductivity and superfluidity are the result of a matter phase change that only occurs when quantum effects can overcome thermodynamic forces.

To understand these forces, physicists investigating superconductivity and superfluidity conduct experiments using ultra-cold temperatures. Techniques to do this having only been in extistence for a decade, but Hulet is an expert at observing what physicists call "many-body phenomena" at temperatures barely above absolute zero. Hulet found that when fermionic lithium-6 atoms are placed in temperatures that have been dropped to within a 30-billionth of a degree of absolute zero, thermodynamic forces become neutralized. As a result, Hulet observed superfluid quantum pairing, where fermionic atoms with equal but opposite spin move in unison, ostensibly behaving as one particle. "The gas behaves as if it is still perfectly paired, which is quite remarkable given the excess of spin-up atoms," Hulet said.

Hulet's team can also change the ratio of spin-up and spin-down atoms with impressive accuracy, but the big surprise came when they found that the superfluid can tolerate a 10 percent surplus of single fermions without detriment. "This was unexpected, and it could signal a new, exotic form of pairing that may also occur in unconventional superconductors or in the quark soup that's predicted to exist at the heart of the densest neutron stars." The team also found that any excess loners above the ten percent threshold initiated a phase change that resulted in the lone fermions being ejected.

As Hulet mentioned, his experiments offer the opportunity for further speculation in regard to very large neutron stars, known as quark stars, which are believed to be composed of a dense superfluid known as strange matter. A quark star could have a mass 5 times greater than the sun compressed into a space smaller than the island of Manhattan.

Source: Rice University