Bending, Breaking and Squeezing Quantum Crystals : 4He and 3He

Orateur : John BEAMISH (University of Alberta, Edmonton, Canada)

Quantum effects dominate the behaviour of helium, to the extent that it remains liquid down to absolute zero and can only be frozen by applying pressure. The Bose liquid, 4He, becomes superfluid below 2.17 K and the Fermi liquid, 3He, has complex superfluid phases at much lower temperatures. Crystals of helium also have unusual behaviour associated with zero point motion, tunnelling, spin and statistics, and I will describe the unique mechanical properties of such “quantum solids”. One of these is “giant plasticity”, an extraordinary softening of the elastic shear modulus due to freely gliding dislocations. Dislocations are also responsible for plasticity and we have recently extended plastic deformation experiments to temperatures below 20 mK. We discovered a transition from steady thermal creep at high temperatures to sudden intermittent slips below 400 mK. These microscopic “quantum earthquakes” reflect dislocation avalanches which are undamped at low temperatures and can occur in a fraction of a microsecond, emitting sound waves at MHz frequencies. Finally, I will describe current measurements looking at flow thro ugh solid helium, a signature of a possible “supersolid” state in 4He crystals.

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