Superfluid 3He at UltraLow Temperatures

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.

Superfluids are ideal systems to study fundamental physics. The lack of viscosity and thermal excitations can often simplify more complex phenomena such as in turbulence and phase transitions. Superfluid 3He has exotic properties associated with macroscopic spin and orbital coherence, and has analogies in superconductivity, particle physics and cosmology. Thus the scientific impacts spread far beyond the immediate fields of ultralow temperature physics and quantum fluids. Our research on cosmological analogues continues to inspire multidisciplinary activity in using condensed-matter experiments to probe cosmological processes. This proposal will promote further activity and potentially open new avenues for fundamental research. Our work on the superfluid A-B phase transition will test general properties of phase transitions and will probe new mechanisms. Likewise, our work on defect creation during interface collisions may produce synergies with many other fields. An understanding of the A-B interface will contribute towards a better understanding of quantum systems in general, with significant long term impacts as quantum systems play an increasingly central role in technology and industry.

The proposed research will benefit cryogenic industries. The use of dilution refrigerators and advance cooling techniques is continuing to grow. Through companies such as Oxford Instruments, ICE and Cryogenic Limited, the UK plays a leading role in the supply of cryogenic equipment. Sales of dilution refrigerators are continuing to increase rapidly, and there is a continuous demand for systems to reach lower and lower temperatures. Equally important is the thermometry needed to measure lower temperatures. The development of new cooling technology and thermometry is an integral part of our research. In the longer term, this fuels advances in commercial technology.

We are leading partners of the MICROKELVIN network which encourages commercial activity both here in Lancaster and in the wider European context. Commercial development is facilitated by Lancaster Cryogenics Limited, a new campus-based company, founded by members of our low temperature group to export knowledge and specialist ultra-low temperature equipment. Our department has a full-time industrial liaison officer who will help to better disseminate our work to industry and to exploit new opportunities for collaboration.

We disseminate our research far beyond our immediate beneficiaries thereby providing cultural enrichment for a wide audience. Our low temperature laboratory is showcased in our extensive outreach programs. We give many low temperature demonstrations, talks at local schools and public lectures. We have 1000+ visitors each year including perspective undergraduate students, parents and school children. Future research funding will allow us to maintain and strengthen this activity. We get extremely good feedback indicating that we have a significant impact, inspiring more students to take up University education, particularly in physics. The resulting higher levels of education benefit society as a whole.

Staff and students working on the project will develop a very broad range of skills including experiment design and construction, vacuum systems, materials processing and techniques for high magnetic fields. They develop generic skills in experimentation, data handling, measurement and analysis techniques. They also develop specialist skills in handling sophisticated research equipment, including state-of-the-art dilution refrigerators and nuclear demagnetization systems. A broad range of skills is attractive to all employment sectors.

Our group has a strong international reputation for building world class infrastructure, for performing novel experiments and for developing new techniques at the lowest temperatures. This adds to the overall profile of UK research and its reputation for pushing the boundaries of what is technically possible.