Damage and annealing in two-dimensional Coulomb crystals

Experiments have shown that the attenuation of shear modes in a 2D classical Coulomb crystal, formed from helium ions trapped below the free surface of superfluid helium, is sensitive to damage in the crystal, and it can therefore be used to monitor the annealing process that follows deliberate damage to the crystal. The characteristic time required for annealing in a circular crystal of radius 13 mm is found to be of order 500 s, and it rises only slowly with decreasing temperature. It is suggested that such behaviour requires the production of configurations of dislocations that can disappear only through dislocation climb, which requires the absorption and emission of vacancies or interstitials, and that the relatively small temperature dependence implies that there must be some source of vacancies or interstitials within the crystal even at the lowest temperatures. (orig.)