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Abstracts
O2.5
Nonlinear ultrasound propagation in solid
4
He due to pinning and
unpinning of dislocations by
3
He
Iwasa I.(1), Kojima H.(2)
1) Faculty of Science, Kanagawa University, Kanagawa, Japan
2) Serin Physics Laboratory, Rutgers University, New Jersey, USA
Ultrasound attenuation (
α
) and velocity (v) at 9.6MHz are measured in
polycrystalline hcp
4
He. Ultrasound signal above 200mK is linear and understood
in terms of resonant vibration of dislocation segments pinned at network nodes
with average pinning length (4.5
µ
m), much shorter than that from shear modulus
(59
µ
m). Dramatic changes in
α
and v are observed below 200mK. The changes
are strongly dependent on temperature, nonlinear and hysteretic. These effects
result from pinning and unpinning of dislocations by
3
He impurities (0.3ppm).
The dislocation damping constant due to thermal phonons, the binding energy
between dislocation and
3
He, and the stress-induced unpinning process are
analysed.
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