QFS 2016 Book of Abstracts

Abstracts

O2.6 Atomic force microscopy of solid He Ori Scaly, Almog Danzig, and Emil Polturak,

Technion- Israel Inst. Of Technology, Department of Physics, Haifa 32000, Israel Crystallites of solid He can move in relation to each other inside the solid. We detected such motion directly, using an in-situ acoustic sensor [1]. An interesting question is what physical mechanism would enable such motion. One possibility is a fluid monolayer at the interface, acting as lubricant. Another possibility is slip of crystal grains induced by gliding dislocations. To answer this question, we constructed a new sensor made of 1micron thick conducting wire loop embedded in the solid. Magnetic flux is applied through the loop. Motion of the wire induced by moving solid will produce a current which will be detected by a SQUID amplifier. The S/N of this apparatus should be 100 times higher than before[1]. We hope to detect the motion of the solid in real time. 1. E. Livne, et al., J. Low Temp. Phys. 180,185 (2015). O2.7 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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