Dr David Bradley

Honorary Researcher

Research Interests

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.

We routinely 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 2mK dilution refrigerator which, using additional adiabatic cooling techniques, can produce liquid 3He temperatures as low as 110 microkelvin. All our ULT apparatus is designed and constructed here in Lancaster. I have over 30-years personal experience of building and using dilution refrigerators.

Recent interests include investigating quantum turbulence produced by vibrating objects in superfluid 3He. 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 viscosity. 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 recently directly measured the energy released by the decay of turbulence at temperatures approaching T=0 by thermal means - the first time this turbulence dissipation energy had been measured in any fluid.

We are also studying the layers of solid 3He that form on the surfaces of any solid at low temperatures, in particular the solid on the strands of aerogel, by thermal and NMR techniques. We found that the solid is very strongly coupled to the surrounding liquid. The paramagnetic nature of the solid spins allowed us to use the solid as in refrigerant. Liquid 3He temperatures below 80 microkelvin were produced in this way.

We showed, for the first time, that superfluid 3He could be used as a particle detector, detecting individual neutron and gamma ray interactions in a small box of superfluid 3He at ~110 microkelvin. This technique may have applications to the detection of Dark Matter (Wimps) due to the very small excitation energy of the superfluid. This work has since been taken forward by another group funded by the EU.

Repetitive creation of single vortices by a vibrating wire in non-rotating superfluid 3He-B has been observed leading to spectacular features 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 - a model that others have since comfirmed by computer simulations.


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.