Focus: Odd Particle Out
A new state of matter that combines the properties of a superfluid and a regular fluid may be within experimental reach. Critics have argued that this theoretical state is unstable, but researchers report in the 14 January PRL that it can exist if the forces holding the particles together have the right properties, which might happen in clumps of ultracold atoms. If found, this strange state could shed light on the behavior of neutron-star cores.
Electrons in a solid can form a superfluid that flows without friction and is called a superconductor. In this state, electrons of opposite “spins” pair together. The two spins occur in equal numbers, so every electron can find a partner. But the small amount of heat that exists in even the coldest materials splits up some of the pairs, and the superfluid is corrupted by a flow of normal, unpaired electrons.
About two years ago, Frank Wilczek, of the Massachusetts Institute of Technology, and Vincent Liu, now at the University of Pittsburgh, proposed a new state of matter which they called breached-pair superfluidity . Unlike conventional superfluids, unpaired particles are a fundamental component of this state, and they exist alongside paired particles, even theoretically at absolute zero.
Breached-pair superfluids have unpaired particles because there is more of one type of particle than the other, so some are left over–like a college sorority dance where some of the young women have to dance on their own due to a lack of men. But the single women mix thoroughly with the dancing couples because breached-pair superfluidity is uniform throughout. In conventional superfluids, the normal component doesn’t completely mix.
Some researchers criticized the theory, saying that the breached-pair superfluid state would not be stable [2, 3]. Now Wilczek’s team has modified the theory, showing that the state can be stable if conditions are right. The original theory assumed for simplicity that the attraction existed only if the particles were touching, as if the dance partners needed to be cheek-to-cheek. But they found that the theory worked much better if instead they allowed the particles to interact at a moderate distance, about arm’s length for the couples. But if the forces act at a much greater distance, another problem arises: The unpaired women attract men from the fraternity next door–new particles enter the system–and then everyone is paired, as in a conventional superfluid.
If the particles in the breached-pair superfluid are electrons, the paired particles will flow as a supercurrent, and the unpaired ones as a simultaneous regular current. If the particles are uncharged, the state could behave something like mixtures of helium-3 and helium-4, whose partial superfluidity is exploited in dilution refrigerators, which cool samples for low-temperature physics experiments. Extending the theory to quarks could help illuminate the nature of the insides of neutron stars.
The researchers say this state could be seen in systems of ultracold lithium atoms prepared in two different spin states. Experimentalists can use laser light and other external fields to control the number of atoms in each state and the distances over which they interact through magnetic attraction and so-called van der Waals forces. “The cold-atom people have been making progress remarkably fast,” says Wilczek. “I wouldn’t be shocked if it were found within five years, and I’d be disappointed if it weren’t discovered within ten.”
“It’s a pretty bizarre state, with a bunch of unusual properties,” says Paulo Bedaque of the Lawrence Berkeley National Laboratory, one of the researchers who criticized the original theory. He agrees that the new formulation is correct. “There’s a chance that experimentalists will be able to make it,” he says.