Response of Solid 4{sup left}He to External Stress: Interdigital Capacitor Solid Level Detector and Optical Interferometer

摘要

Two experiments are being conducted to observe the liquid/solid interface of 4{sup left}He near 1 K. Interesting instabilities are expected to occur when the solid is non-hydrostatically stressed. (1) A compact interdigital capacitor is used as a level detector to observe solid 4{sup left}He to which stresses are applied externally. The capacitor consists of 38 interlaced 50 μm wide and 3.8 mm long gold films separated by 50 μm and deposited onto a 5 mm by 5 mm sapphire substrate. The capacitor is placed on one flat end wall of a cylindrical chamber (xx mm diameter and xx mm long). The solid is grown to a known height and a stress is applied by a tubular PZT along the cylindrical axis. The observed small change in height of the solid at the wall is linearly proportional to the applied stress. The solid height decreases under compressive stress but does not change under tensile stress. The response of the solid on compressive stress is consistent with the expected quadratic dependence on strain. (2)Interferometric techniques are being developed for observing the solid 4{sup left}He surface profile. A laser light source is brought into the low temperature region via single mode optical fiber. The interference pattern is transmitted back out of the low temperature apparatus via optical fiber bundle. The solid 4{sup left}He growth chamber will be equipped with two PZT's such that stress can be applied from orthogonal directions. Orthogonally applied stress is expected to induce surface instability with island-like deformation on a grid pattern. Apparatus design and progress of its construction are described.