  We observe a structural phase transition between two configurations of a
superradiant crystal by coupling a Bose-Einstein condensate to an optical
cavity and applying imbalanced transverse pump fields. We find that this first
order phase transition is accompanied by transient dynamics of the order
parameter which we measure in real-time. The phase transition and the
excitation spectrum can be derived from a microscopic Hamiltonian in
quantitative agreement with our experimental data.
