Double first for superfluids
Mar 29, 2000
The properties of superfluids have fascinated physicists from the time of Landau and Feynman - and continue to do so. Now an international team of physicists from Helsinki, Leiden, Moscow and Grenoble have observed a double-quantum vortex in superfluid helium-3 for the first time (Nature 404 471). In addition to its interest to low-temperature physicists, the work could also be relevant to the cosmic strings that have been proposed by some cosmologists.
Helium-3 flows without friction when it is cooled below the superfluid transition temperature of 2.6 millikelvin. In the superfluid state the helium-3 atoms, which are fermions and therefore obey the Pauli exclusion principle, form Cooper pairs which obey Bose statistics. Since the pairs do not have to obey the exclusion principle, they can all occupy the same quantum ground state and can therefore be described by a single wavefunction or "order parameter". This is what gives helium-3 its unusual superfluid properties.
One property of the superfluid state is that vortices formed by stirring or disturbing the superfluid are quantized. Moreover, the phase of the order parameter must change by an integer multiple of 2p around the core of the vortex. Previously only single vortices in which the phase changes by 2p have been observed. Now the Helsinki-Leiden-Moscow-Grenoble team has used nuclear magnetic resonance to observe double vortices in which the phase changes by 4p. Furthermore, when the superfluid helium-3 is slowly set into rotation under suitable conditions, only double vortices - which are about 10 microns in diameter - are formed.