If a semiconductor at low temperature receives a pulse of photons whose energy is larger than the band gap, the electrons and holes produced by the pulse are sometimes bound in neutral pairs, or “excitons.” A few years ago several groups of investigators found that in some materials at sufficiently low temperatures, excitons, which normally behave like atoms in a gas, condense to form drops of liquid (see PHYSICS TODAY, December 1973, page 17). Recent work has clarified the nature of these drops and has illuminated the interaction between the drops, phonons and other features of the crystal lattice. Much of this work was discussed at the Edinburgh Semiconductor Conference in September.

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