HELIUM CRYSTALS AS A PROBE IN MATERIALS SCIENCE

We review here a number of properties of helium crystals which are par adoxically classical, not quantum. We believe that the study of these properties are of general interest in materials science. Among them ar e the roughening transition and other properties of steps and terraces on hcp helium 4 crystals. We also describe the stress induced instabi lity of helium crystal surfaces. We eventually mention other general p roblems whose study is interesting in helium because its extreme purit y and quasi-infinite thermal conductivity offer unique opportunities f or an accurate control of the relevant physical parameters.