Effect of Crystal Quality on HCP-BCC Phase Transition in Solid 4He

The kinetics of HCP-BCC structure phase transition is studied by precise pressure measurement technique in 4He crystals of different quality. An anomalous pressure behavior in bad quality crystals under constant volume conditions is detected just after HCP-BCC structure phase transition. A sharp pressure drop of 0.2 bar was observed at constant temperature. The subsequent pressure kinetics is a non-monotonic temperature function. The effect observed can be explained if we suppose that microscopic liquid droplets appear on the HCP-BCC interphase region in bad quality crystals. After the interphase region disappearance, these droplets are crystallized with pressure reduction. It is shown that this effect is absent in high quality thermal-treated crystals.Comment: 4 pages, 4 figure

NEW PHENOMENA RELATED TO PROPAGATION OF WAVE PROCESSES IN He–II FLOW

On étudie les effets d'une perturbation alternative sur les écoulements stationnaires d'He–II dans les films et les capillaires. L'étude de la dépendance en fréquence des effets obtenus permet de donner une estimation des temps caractéristiques associés aux processus dissipatifs présents dans ces écoulements. Les résultats expérimentaux sont en bon accord avec un modèle théorique basé sur une force dissipative non-linéaire en vitesse.Specific features of steady superfluid He–II flow modulated by a wave process have been studied in He II-films and narrow channels. The frequency dependence of the effects permits the characteristic times of the dissipative processes developing in superfluidity to be estimated. The experimental results are in agreement with the theoretical model based on a nonlinear behaviour of the dependence of friction force on superflow velocity