  Coulomb crystallisation of large ensembles of ions has in the past years been
intensively studied experimentally with many spectacular results of relevance
to infinite systems in one-, two-, and three-dimensions.While strings of a few
ions have proven to be very attractive objects in quantum information
processing, larger Coulomb crystals have very recently found applications
within other aspects the dynamics of quantum systems. Smaller finite ensembles
of cold identical charged particles confined by a harmonic potential
furthermore constitute very special types of clusters due to the pure repulsive
long-range inter-particle forces. Here, we report on the direct imaging of
metastable structures of Coulomb clusters consisting of a few thousands
confined and laser-cooled 40Ca+ ions. The observations are attributed to
structural excitations due to finite temperatures, a feature likely to appear
in clusters of short-range interacting particles, but yet not observed
directly.
