OPTICAL OBSERVATION OF GROWTH AND MELTING DYNAMICS OF HE-3-HE-4 MIXTURE CRYSTALS

Crystals grown from 2.1% and 9.3% solutions of He-3 in He-4 were obser ved in, a cryostat with optical access from two orthogonal directions (above and from the side). Video films of dynamic processes were recor ded at temperatures between 0.5K and 1.8K. Facetted melting of crystal s (in. contrast with rounded melting which is the general case) was ob served below 0.75K. This is in, accordance with previous experiments w hich showed a higher He-3 concentration. along the edges of the crysta l. We also recorded morphological growth instabilities of the 9.3% b.c .c. crystals. In the temperature region below about 0.9K where the lat ent heat is negative no dendritic growth was observed, which we interp ret as resulting from opposing effects of temperature and concentratio n gradients.