Experimental Condensed Matter Physics
Ultra-Low-Temperature Physics
(Professors A M Guenault and G
R Pickett; Drs D I Bradley, S N Fisher, R P Haley and P Skyba)
Liquid 3He becomes superfluid only when it is cooled to 1-2
mK. We are studying the properties of the superfluid well below the transition
temperature Tc, in a regime in which the normal
fluid fraction is only about 10-5. The mean free path
in the fluid becomes very long, and the quasi-particles behave ballistically.
These experiments require the coldest possible temperatures, and we are using
nuclear demagnetisation of copper spins to reach around 100 µK in the
liquid. The properties of the fluid are probed using the mechanical resonance
of a vibrating wire. Four major experiments are in progress: (i) properties of
pure liquid 3He-B in the ballistic limit, thermal
boundary resistance, Landau critical velocities in 3He-B, non-linear quasiparticle damping and long-lived spin
precession; (ii) behaviour of 3He in dilute solutions
in 4He, (iii) the static and dynamic properties of the
A-B boundary in superfluid 3He and properties of the A
phase at temperatures much lower than Tc; (iv)
sudden local heating due to neutron absorption, to investigate phase
transitions such as those that may have created cosmic strings in the early
universe. Experiments are also in progress searching for superfluidity in 3He-4He solutions, cooling
metal samples (achieving 7 µK - a probable world record), and using
superfluid 3He as a dark-matter detector.
Further details
Superfluid 4He
(Professor P V E McClintock; Drs P C Hendry and N S Lawson)
Three groups of experiments are being developed, using Lancaster-prepared isotopically pure 4He (a million times purer than commercial 4He): (i) using a fast adiabatic expansion through the lambda (superfluid) transition to model the possible production of cosmic strings in the early universe through the Kibble mechanism; (ii) using small spherical objects (negative "ions") to study several aspects of 4He superfluidity, including vortex creation in ultra-dilute superfluid 3He-4He solutions, and roton creation by exotic ions; (iii) using a new computer-controlled dilution refrigerator to investigate quantized vortex decay mechanisms in the mechanical vacuum below 100 mK.
Further information.
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