An important theme that threads through many areas of current interest in condensed matter physics is the effect of randomness and disorder. Prior to the 1960s, disorder and impurities were often viewed as unavoidable nuisances that masked the true behavior of ideal systems. We have since learned that disorder itself can bring forth fascinating and often unexpected new phenomena in condensed phases of matter. (See the December 1988 special issue of PHYSICS TODAY, dedicated to disordered solids.)
A minute amount of appropriately placet impurity has fascinating effects on the nature of the superfluid transitions in helium‐4 and helium‐3, and on the ordering of 3He‐4He, mixtures.