S. G. Lipson

Observation of macroscopic structural fluctuations in bcc solid {sup 4}He

We report neutron diffraction studies of low density bcc and hcp solid {sup 4}He. In the bcc phase, we observed a continuous dynamical behavior involving macroscopic structural changes of the solid. The dynamical behavior takes place in a cell full of solid, and therefore represents a solid-solid transformation. The structural changes are consistent with a gradual rotation of macroscopic grains separated by low angle grain boundaries. We suggest that these changes are triggered by random momentary vibrations of the experimental system. An analysis of Laue diffraction patterns indicates that in some cases these structural changes, once initiated by a momentary impulse, seem to proceed at a constant rate over times approaching 1 hour. The energy associated with these macroscopic changes appears to be on the order of kT. Under similar conditions (temperature and pressure), these effects were absent in the hcp phase.

Dynamics of fluid bubbles in3He-4He mixture crystals

We report visual observations of crystallization and melting of3He-4He mixture crystals containing several percent3He in equilibrium with the fluid. At temperatures near the minimum of the melting curve, a phaseseparation of some sort occurs in a crystal in its coldest part, macroscopic fluid bubbles appear and can be seen to move through the crystal at a measurable velocity in the direction of the temperature gradient. This phenomenon looks very similar to boiling, but occurs within the solid. We discuss the dependence of the density, size, and velocity of the bubbles on the magnitude of the temperature gradient and relate the findings to the phase diagram of the mixtures.

USE OF HOLOGRAPHIC INTERFEROMETRY FOR STUDIES OF PHASE CHANGES IN HELIUM NEAR THE ABSOLUTE ZERO

ABSTRACT We are using real-time and double-exposure holographic interferometry to study the properties of 3He-4He mixtures at very low temperatures, as a function of temperature, pressure and 3He concentration. The effects so far investigated are diffusion, surface tension and crystallization. The experimental method is described together with several examples of results obtained.