Abstract
The mechanical properties of crystals are strongly affected by dislocation mobility. Impurities can bind to dislocations and interfere with their motion, causing changes in the crystal's shear modulus and mechanical dissipation. In 4He crystals, the only impurities are 3He atoms, and they can move through the crystal at arbitrarily low temperatures by quantum tunneling. When dislocations in 4He vibrate at speeds v < 45 μm/sec, bound 3He impurities move with the dislocations and exert a damping force B3v on them. In order to characterize 4He dislocation networks and determine the 3He binding energy, it is usually assumed that B3 is proportional to the concentration of 3He bound to the dislocations. In this preliminary report, we determine B3 in a crystal with 2.32 ppm 3He and compare with our previous measurements of B3 in natural purity 4He crystals to verify the assumption of proportionality.
Highlights
Recent work[1, 2, 3, 4, 5] has demonstrated that the mechanical properties of hcp 4He crystals can be understood in great detail by modeling their dislocations as elastic strings that vibrate between pinning points[6]
At low dislocation speeds and low temperatures, 3He impurities bind to the dislocations and move with them, damping their vibrations[3]
The binding energy EB was determined from the frequency dependence of the peak dissipation temperature under the assumption that the damping due to 3He is proportional to the concentration of 3He atoms bound to the dislocations [3, 5, 7]
Summary
Recent work[1, 2, 3, 4, 5] has demonstrated that the mechanical properties of hcp 4He crystals can be understood in great detail by modeling their dislocations as elastic strings that vibrate between pinning points[6]. The binding energy EB was determined from the frequency dependence of the peak dissipation temperature under the assumption that the damping due to 3He is proportional to the concentration of 3He atoms bound to the dislocations [3, 5, 7].
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