Experiments on the growth of hep helium crystals from dilute ^He-'^He solutions show that a ^He concentration on the order of 10 ~^ is sufficient to lower the roughening temperature of the c facet by 20%. We interpret this as resulting from the adsorption of up to 0.25 monolayer of ^He on the liquid-solid interface, with a binding energy of about 10 K. The presence of ^He leads to a growth instability near 1 K.

At a temperature below the roughening temperature of a facet on a crystal, the junction between the smooth planar facet and the adjacent "rough" rounded surface should show a characteristic critical behavior, which has been calculated for several different models of surface structure. This paper presents an experimental investigation of such junction regions in hcp He crystals next to the c facets (0001). It was found that the rough surface curves away from the plane of the facet as x~, where P=1.55+0.06. This result agree with the predictions of various terrace-step-link types of model, but disagrees with the mean-field model of Andreev.

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