Rotating Superfluid 3He
Peter Berglund 7.2.2000
3He is a unique system of condensed matter physics - the equivalent of hydrogen in atomic physics. In spite of its simple structure, at low temperatures liquid 3He condenses to fermionic superfluid states which display the most extreme examples of symmetry breaking. The complexity of the 3He superfluids can be compared with the physical vacuum of unified field theories. The topological objects in the order parameter field of superfluid 3He - such as textural point defects, quantized vortex lines, and domain walls - are in many respects similar to the strings and other topological defects in relativistic quantum field theories. An important difference is that in high energy physics these objects are still hypothetical, while in the case of superfluid 3He they can be investigated experimentally.
In the Low Temperature Laboratory (LTL) the 3He superfluids have been studied since their discovery in 1972. In 1978, the Finnish-Soviet ROTA collaboration was established for investigating the 3He superfluids in rotation. The first cryostat for this work was completed in 1981 and the second in 1988. Most of our research has focused on the structure of the rotating states in these superfluids with multi-component order parameter fields. The bulk of our work can be characterized as dealing with the structure, nucleation, and dynamics of topological defects in coherent systems, a topic which is of current interest in many areas of physics, for instance in field theory and its applications to cosmology. The superfluid phases of liquid 3He provide the most versatile laboratory system for this general purpose. Rotation, in turn, is the most effective means of producing objects of different dimensionalities and structures. It is here that such phenomena as topological confinement, nucleation of singularities, and interactions across phase boundaries can be studied experimentally in great detail and reliably understood on the basis of a powerful microscopic theory. Eight different types of quantized vortices have been discovered in our experiments: one structure in 3He-A1, four in 3He-A2, and three in 3He-B. Each of them represents a novel topological object with peculiar symmetry and structure. Most of the experimentally verified knowledge on quantum vorticity in 3He originates from the Low Temperature Laboratory.
Symmetry breaking in an ordered phase below a phase transition point is a familiar phenomenon in condensed-matter systems such as superfluid 4He, superconductors, liquid crystals, and magnetically ordered spin systems. However, in this regard the 3He superfluids are the most extreme cases owing to the complexity of their symmetry breaking properties, which all can be traced back in great detail to p-wave Cooper pairing in these Fermi systems. A particular advantage of superfluid 3He is its natural purity. This is, by far, the cleanest system known with no bulk impurities; only the container walls represent a disturbance from the ideal. In contrast to superfluid 4He, the topology and structure of the various objects is much richer in 3He, the size of the singularities is two orders of magnitude larger, pinning is weaker, and remnant objects are less of a difficulty.