Buhrman came by one day with a picture of a river delta, formed by sweeping all the oxygen vacancies downstream in a high-T_c constriction in a film. He explained that this was odd, because (a) the field was so small and the diffusion constant at that vacancy concentration so small that the vacancies shouldn't have budged, and (b) the field at which the vacancies started moving seemed rather sharp. Lisa Wickham and I started working on this, focusing on the rapid concentration dependence of the mobility, and found an interesting collective instability to the formation of large voids. We still haven't explained their threshold field, but we're guessing it has something to do with pinning at dislocations.
James P. Sethna, email@example.com
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).