Dr D Ian Bradley
Senior Lecturer
Room: A29 Physics Building
Tel: +44 (0)1524 593074
Fax: +44 (0)1524 844037
Email: i.bradley@lancaster.ac.uk
Research
I am a member of the Lancaster Ultra-Low Temperature Group. My research interests are in the properties of matter at ultra-low temperatures, temperatures below a few thousands of a degree above absolute zero, and the obtaining of such low temperatures. Most of my time is spent investigating the superfluid properties of liquid 3He, the spin 1/2 isotope of helium. The exotic and complex superfluid phases of 3He are only accessible below ~3 millikelvin, in contrast to the more common (spin 0) isotope 4He (which is superfluid below the much higher temperature of ~2K).
We can cool liquid 3He to temperatures of 100 microkelvin and dilute 3He-4He mixtures to below 3 millikelvin. My work is centred around a purpose built 2 millikelvin dilution refrigerator which, using additional adiabatic cooling techniques, can routinely produce liquid 3He temperatures as low as 110 microkelvin. All our ULT apparatus is designed and constructed here in Lancaster.
Recent interests include investigating quantum turbulence produced by vibrating objects in superfluid 3He - see for example D I Bradley et al, Phys. Rev. Lett. 93, 035301, 2006. Quantum turbulence is an ideal system for studying turbulence. In general, turbulence is still rather poorly understood, despite its significance and importance on all length scales. In superfluid 3He at very low temperatures, we can study turbulence in the pure inviscid superfluid without the complication of the viscous normal component. An additional advantage is that the vortex lines all have the same magnitude of circulation - a fundamental property of a quantum fluid - adding a further simplification compared to the vortex lines in a classical fluid. We are currently trying to detect the decay of turbulence at temperatures approaching T=0 by thermal means.
We are also studying the layers of solid 3He that form on the surfaces of any solid at low temperatures. In particular we are studying the solid on the strands of aerogel by thermal and NMR techniques. We find that the solid is very strongly coupled to the surrounding liquid. The paramagnetic nature of the solid spins allows us to use the solid as in refrigerant. Liquid 3He temperatures below 80 microkelvin can be produced in this way.
We demonstrated that superfluid 3He could be used as. We showed, for the first time, that superfluid 3He could be used as a particle detector, a low-energy nuclear recoil detector. We detected individual neutron and gamma ray interactions in a small box of superfluid 3He at ~110 microkelvin (D I Bradley et al, Phys. Rev. Lett. 75, 1887, 1995). This technique may have applications to the detection of Dark Matter (Wimps) due to the very small excitation energy of the superfluid. Follow up work has lead to a better understanding of the detected signal (W M Hayes et al, proc LT21, Czech. J. Phys. 46, 2885, 1996).
Further work at Grenoble (C Bauerle et al, Nature 382, 332, 1996) suggested that neutrons absorbed in the superfluid create large numbers of quantum vortices. The creation of these vortices is believed to be analogous to the creation of cosmic strings in the early Universe, the Kibble mechanism.
Repetitive creation of single vortices by a vibrating wire in non-rotating superfluid 3He-B has recently been observed (D I Bradley, Phys. Rev. Lett. 84, 1252, 2000). These spectacular features are seen on the velocity-force characteristic of the vibrating wire. At certain critical velocities, plateaus are observed. Rapid fluctuations in the wire velocity occur on these plateaus which we ascribe to the production of single vortex loops. The enigmatic feature of these effects is that there appears to be not only a critical velocity for creation but also a critical velocity associated with the decoupling of the vortex from the wire.
Teaching
I have been involved in all aspects of physics teaching. I have taught courses on introductory astronomy and astrophysics, electromagnetism, thermodynamics, low temperature physics and quantum mechanics. Also, as part of the Department's novel Universe as an Art course for non-science students, I have given descriptive lectures on introductory astronomy.
I am Director of the Dame Kathleen Ollerenshaw Observatory. In this role, I manage the equipment in the observatory and supervise undergraduate student projects on observational astronomy. When time permits, I also observe using the observatory, or with my personal telescope. Several ideas for undergraduate projects have been trialled in this way.
Non-Academic Interests
My main non-academic interest outside of my family are mountaineering, in particular rock climbing, ice climbing and mountain walking and astrophotography. I am a keen member of the Preston Mountaineering Club and the Eddington Astronomy Society in Kendal. From mountaineering, I developed an interest in rocks and the effect of the local geology on the landscape. I particularly like visiting regions of the world with interesting (and active!) geology e.g. volcanic and glacial regions.