Superfluid 3He at UltraLow Temperatures

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Superfluid 3He has many exotic properties and provides a model system for investigating fundamental processes. It is the only accessible material which has absolute purity. It supports dissipationless flow of mass, spin and orbital angular momentum which give rise to a variety of macroscopic coherence phenomena. The coherent orbital and spin properties are described by vector order parameters which vary on very large length scales, limited only by the size of the experimental cell. Our group specialises in novel cooling and measurement techniques at ultralow temperatures. We routinely cool superfluid 3He to record low temperatures where the few remaining thermal excitations are highly ballistic and the superfluid is essentially in its groundstate.We plan a series of experiments to study fundamental properties of phase transitions and interfaces. The A-B phase transition at low temperatures occurs at a fairly large magnetic field of order 0.4Tesla. Using custom made superconducting solenoids we can accurately control the magnetic field profile to stabilise and manipulate the A-B phase interface. We will incorporate techniques to measure very small, femtowatt, energy dissipations when the interface moves. We will oscillate the interface to investigate how the different dissipation mechanisms depend on the velocity amplitude and the frequency. At low velocity/frequency the interface dissipates energy by scattering thermal excitations. At higher velocities/frequencies dissipation may occur by generating new excitations via processes analogous to particle production in high energy physics. We also expect the motion to induce interesting dynamics of the orbital textures on both sides of the interface.The transition from A-phase to B-phase on cooling presents a long outstanding puzzle. According to standard theories the transition should not occur, even on astronomical timescales. Experiments show that the transition occurs quite readily. A possible explanation has been proposed based on `resonant tunneling' between metastable states. We will perform controlled experiments to test this. By forming a sharp magnetic field minimum we will shape the B-phase nucleating region into a bubble remote from the cell walls to eliminate surface mechanisms. According to the resonant tunneling model, the transition should only occur at particular temperatures and magnetic fields, giving a clear experimental signature.We will construct an aerogel-based 3He cooling stage to access lower temperatures. We will directly cool, by demagnetization, the nano-network of solid 3He layers which coat the aerogel strands. These layers are in extremely good thermal contact with the surrounding liquid owing to rapid exchange. The stage will have an enclosed cavity to enable experiments on bulk superfluid 3He. The cavity will be completely surrounded by cold demagnetised aerogel to eliminate heat leaks. We believe it is possible to reach a new temperature regime where there are only a few remaining thermal excitations in the entire volume. We will use Nuclear Magnetic Resonance to simultaneously probe the bulk superfluid, the aerogel-confined fluid and the nanometer solid 3He layers. This will enable us to explore new phenomena in the extreme low temperature limit. We will study ultralow temperature magnetic phase transitions in the solid layers. In the bulk B-phase we will investigate the Persistent Precessing Domain (PPD), a Bose-Einstein condensate of magnons. The free decay of the PPD may last several hours at the lowest temperatures. Under such extreme conditions, additional dissipation mechanisms may emerge, such as from ionisation tracks left by cosmic rays.The research will lead to a better understanding of fundamental processes in quantum systems, phase transitions and phase interfaces, as well extending the capabilities of cooling technology.