Novel Experiments in Multiphase Superfluid 3He at Ultralow Temperatures

Superfluids, superconductors and Bose condensed dilute gases are extremely interesting since their constituent atoms form coherent ground states, in which the behaviour of the whole system is correlated, leading to macroscopic quantum phenomena.However, superfluid 3He is unique due to its multi-component order parameter. The 3He atoms form pairs which have both spin and orbital angular momentum. The mass, spin and orbital motions each exhibit coherent quantum behaviour giving rise to a whole range of exotic properties. This added complexity allows superfluid 3He to exist in two very different coherent phases, A and B, depending on the temperature and magnetic field.We are able to cool superfluid 3He to temperatures where virtually all atoms are paired, so we have an almost pure quantum state. By applying a suitably shaped magnetic field profile we can stabilise different phases in different regions, with various geometries including an isolated bubble of B-phase surrounded by A-phase. The bubble geometry will allow us to study fundamental processes, which might otherwise be influenced by walls, such as phase nucleation, thermal transport and turbulence.The A-B interface is a coherent structure separating two highly coherent phases. The order parameter must make a complex pirouette in crossing from one phase to the other, matching-up smoothly the mass, spin and orbital degrees of freedom. This unique system gives us an entre into a wide range of new physics. It is clearly an interesting system in its own right. However, it also provides a model for less accessible systems. For example, it is the nearest thing we have in the laboratory to a cosmological brane (equivalent structures in space-time). By colliding two A-B boundaries, we can simulate brane-annihilations in the laboratory. These are of fundamental interest to the braneworld scenarios of cosmology.By immersing aerogel in 3He we can study the superfluid phases in the dirty limit generated by the disorder induced by the nanometre sized silica strands in the aerogel. Furthermore, when we immerse the aerogel in superfluid, a few atomic layers of 3He atoms are adsorbed onto the silica strands to make a substantial solid 3He component in the helium-aerogel system. The solid 3He is highly magnetic and is in intimate contact with the fluid. This provides a unique opportunity for using magnetic cooling techniques on the solid and the near perfect thermal contact to cool the superfluid. The solid layers around the silica strands form a system of 3He-nanotubes which also have potential for revealing new exciting physics. We intend to develop a 3He-aerogel cooling stage to study both the solid 3He-nanotubes and to cool the superfluid to new a low temperature regime where there are essentially no thermal excitations over macroscopic volumes of the liquid.Recently, we have also found that superfluid 3He provides a particularly useful tool for studying quantum turbulence at low temperatures. Quantum turbulence is essentially a tangle of quantum vortex lines (line defects around which the superfluid circulation is quantised). Quantum turbulence has close analogues with classical turbulence but is much simpler, due to the quantised vortices and lack of viscosity, and might therefore provide better insights to understanding turbulence in general. We plan to use highly sensitive calorimetric techniques, which we have developed earlier, to measure the energy decay of quantum turbulence in the zero temperature limit, providing better information on fundamental decay mechanisms.Finally we are using these experiments to pilot the development of a new form of the highly sensitive quartz resonator, custom-designed to maximise its interaction with superfluid 3He at the lowest temperatures. We hope that this will replace the currently ubiquitous vibrating wire resonator (also developed at Lancaster) as the standard low temperature quantum fluids sensor/thermometer.