Nailing Down the Exciton in LiF SEPTEMBER 17, 2008 Excitons are
worth getting excited about. For one thing, they are a key
element in the functioning of semiconductors and insulators, and
understanding their structure and how they form and behave in
different materials is vitally important to the development of
new materials and technologies. For another, the true nature of
a particular exciton has been the subject of controversy in the
materials science community for 70 years. Now, experimenters
using two x-ray beamlines at the U.S. Department of Energy’s
Advanced Photon Source (APS) at Argonne National Laboratory have
settled the controversy, while setting the stage for research
that could help improve materials used for alternative energy
sources. An exciton is a fascinating thing: an electron bound to
an electron hole, acting as a single entity. But their short
lifespan makes them maddeningly elusive and hard to
characterize. They come in two basic varieties: the Wannier
exciton, which is large, with a fr