Research
Earlier Research
Highlights of earlier research by the He-4 group include
measurement of the Landau critical velocity, demonstration that
creation of quantized vortices involves macroscopic quantum tunneling, the world record for the purest He-4, the first successful storage of ultra-cold neutrons in He-4, and pioneering experiments on quantum turbulence in the low temperature limit.
Gold-plated cell and electrodes used to measure the Landau critical velocity in superfluid He-4
Quantum Turbulence
Richard Feynman once referred to turbulence as the "last unsolved problem in physics". It is a fascinating problem, and of near-universal importance.
We are investigating the creation and decay of quantum turbulence (QT) - i.e. tangles of quantized vortex lines - at mK temperatures. The QT is created either by an oscillating grid or, as we plan, by a grid drawn through the liquid by a levitated, superconducting, linear, stepper motor. Possible methods of detection include ion-capture (as used in our initial experiments), calorimetry, or changes in the frequency or amplitude of oscillating forks or wires.
The work involves collaboration with Professors William (Joe) Vinen (University of Birmingham), Gary Ihas (University of Florida) and Ladislav Skrbek (Charles University, Prague).
Wave Turbulence
In addition to the familiar manifestations of vortex turbulence in bulk fluids, sketched and discussed by Leonardo da Vinci, turbulence also occur in systems of waves in many physical contexts.
Modeling wave turbulence (WT) with surface waves on liquid hydrogen, and with second sound (entropy-temperature) waves in bulk superfluid helium, we have demonstrated the celebrated Kolmogorov scenario: a flux of energy towards successively smaller lengthscales until it is finally dissipated by viscosity. We have recently discovered, however, that wave energy can sometimes flow in the opposite direction, towards larger lengthscales. It is this result that provides a possible creation mechanism for oceanic "rogue waves".
We are currently following up these discoveries in more detail. We will extend the second sound work to elevated pressures, and will investigate the effect of He-3, both of which alter the nonlinearity coefficient of the superfluid and may be expected to modify the energy cascades. We are also planning small-angle neutron scattering experiments at ISIS (Rutherford Appleton Laboratory), hoping to excite WT on the superfluid surface.
The work involves close collaboration with the Institute for Solid State Physics in Chernogolovka, near Moscow, as well as with ISIS.
Funding
The research is funded by the Engineering and Physical Sciences Research Council (UK). Our current grants total over \pounds 1.1M, including EPSRC/NSF World Materials Network funding held jointly with Birmingham and Florida, and an EPSRC/STFC neutron scattering grant under the Next Generation Facility Users Programme. We hold a Royal Society Joint Project grant with our collaborators in Chernogolovka.