Exotic Phenomena in Superfluid 3He at Ultralow Temperatures

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Superfluid 3He is the most exotic liquid in existence. It only exists below a few thousandths of a degree above absolute zero. It owes its existence entirely to quantum mechanics and is therefore interesting to study to promote the better understanding of quantum systems in general. While it is superfluid and may flow without friction, the fact that it also has an associated spin 'superfluid' and an orbital angular momentum 'superfluid' gives it many unique properties, many of which remain to be discovered. Our group has pioneered the study of ballistic heat transport in superfluid 3He. Heat in the superfluid is carried by quasiparticle excitations. At very low temperatures these are so few as to hardly ever scatter. We can thus generate beams of ballistic quasiparticles and fire them at various obstacles formed by the superfluid itself. We have so far mastered the techniques for observing the Andreev reflection of such beams (a form of reflection unique to superfluids and superconductors). We now wish to develop the methods needed to measure the transmission of quasiparticle beams.We will use this technique to investigate the decay of quantum turbulence. In superfluid 3He turbulence takes the form of a tangle of identical quantised vortex lines. This is much simpler than classical turbulence which has eddies/vortices of variable sizes. The study of superfluid turbulence will give us a better understanding of turbulence in general. With quasiparticle transmission techniques we hope to obtain more quantitative information on the turbulence decay mechanisms in the zero-temperature limit where the dissipation mechanism should be determined by quantum effects rather than by conventional viscosity. We will also investigate the superfluid phase diagram of dirty 3He. Impurities may be effectively added to liquid 3He by confining it in aerogel, a nanoscale network of silica strands. Since pure 3He is so well understood, it is the ideal substance for investigating such effects. We will study how impurities influence the various superfluid phases and the transitions between them. Gapless superfluidity is an exotic phenomenon common in dirty superconductors where the binding energy of the constituent pairs providing the superfluid behaviour vanishes. We have recently seen this behaviour in the thermal conductivity of superfluid. With similar techniques we plan to study the cross-over from gapless to (the usual) gapped superfluidity. Further, where the superfluid transition in aerogel occurs at zero temperature, this constitutes a quantum phase transition dominated by quantum effects. This seems to be the cleanest such transition known and we are well placed to investigate the associated quantum fluctuations.The superfluid orbital properties of 3He are not evident at the temperatures available to most research groups. Our novel techniques allow us the lowest achievable temperatures where orbital superfluidity becomes apparent. We have indirect evidence of orbital superfluidity from exotic NMR signals from persistent precessing domains (PPDs). These are ultra long lived domains of coherent spin precession which have laser-like properties. By looking at the interaction of two PPDs we hope to gain more direct evidence of orbital superfluidity.Finally we plan to demonstrate an exotic mechanism unique to superfluid 3He affecting the motion of an object in the liquid. At absolute zero there are no excitations and an object should move freely through the superfluid as through a vacuum. However, if the object is heated, the thermal emission of quasiparticles should damp its motion. We will investigate this by moving a heated aerogel sample through the superfluid.We emphasise that most of the proposed experiments are only feasible at the very lowest temperatures made possible by our unique cooling techniques.