Do Solids Flow?

When we slowly spin a cup of tea, we expect the tea to rotate along with the cup. However, superfluids behave differently. In its superfluid state near absolute zero temperature, liquid helium remains rigidly stationary when the container is rotated. Superfluids possess other remarkable properties, such as the ability to creep up the walls of the container. Can solids exhibit similar "supersolid" behavior? Recent experiments by Kim and Chan (Penn State University) suggest that such exotic solids exist. They find that when solid helium is oscillated at very low temperatures, a fraction of the crystal fails to oscillate with the rest. These experiments have stimulated enormous interest because they challenge fundamental concepts of rigidity in solids. Recently, Anderson, Brinkman and Huse proposed tests to help further our understanding of solid helium¹. In the theory, solid helium always contains a finite number of vacancies, or empty crystal sites, as the temperature nears absolute zero (in classical solids vacancies vanish). In the supersolid phase, the vacancies flow like a superfluid. The predictions appear compatible with available x-ray diffraction and heat capacity results. The authors show how improved measurements can provide more rigorous, quantitative tests.