Quasiparticle Imaging and Superfluid Flow Experiments at Ultralow Temperatures

Our group has pioneered novel techniques to cool superfluid 3He to ultra-low temperatures where thermal quasiparticle excitations are highly ballistic, having intrinsic mean-free-paths which approach kilometre length scales. Superfluid 3He displays a wide range of exotic spin, orbital and mass superfluid behaviour, providing an ideal system to study and to gain a better understanding of quantum systems in general. We have developed techniques to generate beams of ballistic quasiparticles and scatter them from various obstacles formed in the superfluid. Superfluid structures such as vortices, textures and phase boundaries have a large cross-section for Andreev reflection (a form of near-perfect retro-reflection of excitations unique to superfluids and superconductors), so are readily probed by excitation beams. We aim to further develop the techniques to build a quasiparticle camera to directly image superfluid structures providing time and spatial resolution to study their dynamics.We will first apply the technique to image the beam of excitations which is emitted by a vibrating wire above some critical velocity. The excitations are created by the wire as it breaks apart the `paired-atom' superfluid condensate. Quantum vortices (line defects in the superfluid with a well-defined circulating flow) are also produced above this critical velocity. The two processes appear to be closely linked, but the mechanism is not understood. The images will allow us to simultaneously observe the pair-breaking beam and vortices, which will provide detailed information about the generation processes.At higher velocity amplitudes, vibrating wires produce quantum turbulence, a complex tangle of vortex lines. Despite its overwhelming importance to a wide range of science and technology, turbulence in general is poorly understood. Quantum turbulence is conceptually much simpler than classical turbulence and is far more amenable to computer simulation. The study of superfluid turbulence may thus eventually provide a better understanding of turbulence in general. There are several interesting unanswered questions to be addressed in quantum turbulence, such as how it develops and decays in the absence of viscous forces at low temperatures. We aim to use the new imaging techniques to directly image quantum turbulence to provide detailed dynamical information for direct comparison with theory.We will also develop a new technique to allow very precise controlled motion of an object in the superfluid to study various flow related properties. In particular, it will allow a comprehensive study of the pair-breaking mechanism over a wide frequency range, including near-uniform (zero frequency) flow. This is interesting since the mechanism is thought to involve the population of bound surface states (including so-called Majorana states which have received a great deal of recent theoretical interest) and there may be analogies with quantum Hawking radiation.The flow device will also allow us to measure extremely low velocities. This will enable us to explore possible supersolid behaviour in solid 4He at low temperatures by measuring the slow motion of a wire through the solid. Supersolidity is a very exotic phenomenon, predicted by some theories, where a quantum solid can exhibit frictionless flow. There is widespread speculation that supersolidity has been observed in 4He at low temperatures, but the observations might also be explained by defects in the solid with superfluid cores. This topic remains highly controversial and has received a great deal of interest. The current device will be very sensitive to both defects and supersolid flow, thus providing key information to resolve this issue.We will also investigate new avenues for research in exotic superfluid phases, spin superfluidity and high frequency/small length scale phenomena. The new techniques developed will be very versatile with a wide range of future applications.

Superfluids are ideal systems to study a wide range of fundamental physics, with close analogies with superconductivity, particle physics and cosmology. So the academic beneficiaries of this research reach far wider than the immediate field of quantum fluids and solids. We have a good track record for exploiting these analogies, and we expect to continue this in the current proposal. A few recent examples of projects spanning wider fields, which have arose as a direct result of our research are: (1) The European MICROKELVIN collaboration (2009), a 4.2Meuro FP7 project to widen access to ultralow temperatures for nano-science, condensed matter physics, cosmology and instrumentation (our group is one of three core access-providing partners with 12 partners in total including a commercial cryogenic business); (2) The ESF COSLAB Network (2001-2006), a 350keuro multidisciplinary initiative to exploit analogies between cosmology and condensed-matter systems. This network was founded on our pioneering work investigating vortex production in rapid phase transitions and analogies with cosmic string creation in the early Universe. This work led to a 300k(ECU) EC-HCM Network Phase transitions in the early Universe (1995-8), followed by a FF516k ESF Network Topological Defects in Particle Physics, Condensed Matter and Cosmology (1997-2000) and then to COSLAB; (3) ULTIMA (2005), a 300keuro dark matter detector project. Our invention and development of `black-body radiator' devices inspired the 2001 MACHe3 project in Grenoble to explore their potential use in making a full-scale dark-matter detection facility, which then led to ULTIMA. The techniques developed in this proposal will have various impacts. The ability to image structures with quasiparticles will be a major step in further developing low temperature technologies, allowing far more detailed studies of a wide range of quantum phenomena. The development of custom tuning forks with our industrial collaborator, Statek, may also lead to wider impacts. To optimise the tuning forks for low temperature applications, we will need to further minimize intrinsic losses in order to maximize sensitivity at the lowest temperatures. Potentially this could lead to improved designs for more conventional uses of tuning forks (e.g. they are widely used in timing devices such as wrist watches) which would then have wider impacts in manufacturing and commerce. The proposed research will also contribute towards a better understanding of quantum systems (superfluids and superconductors) and of turbulence. This could have a very significant impact in the long term, since quantum systems are likely to play an increasingly central role in future electronics industries, and a better understanding of turbulence would have far reaching impacts in, e.g., transport and energy production industries. We are very successful at disseminating our research to a wide audience. We regularly: publish in high impact journals; give invited talks at major conferences (note we will host the main Quantum Fluids and Solids conference in 2012); give research seminars at other institutions; and visit other leading research groups. Our department has a full-time industrial liaisons officer who will help us to better disseminate our work to industry and to exploit any new opportunities for collaborative work. We also disseminate our research to the wider public by our web pages and various outreach activities. We have good contacts with local schools and we regularly give talks, lab tours and demonstrations for school visits, prospective students and their parents. We will continue to develop all of these activities. Our group is well known internationally for developing new techniques and for performing novel experiments at the lowest temperatures. Our leading role in this field, impacts on the overall profile of UK research and its reputation for pushing the boundaries of what is technically possible.