  Collective states in cold nuclei are represented by a wave function that
assigns coherent phases to the participating nucleons. The degree of coherence
decreases with excitation energy above the yrast line because of coupling to
the increasingly dense background of quasiparticle excitations. The
consequences of decoherence are discussed, starting with the well studied case
of rotational damping. In addition to superdeformed bands, a highly excited
oblate band is presented as a new example of screening from rotational damping.
Suppression of pair correlation leads to incoherent thermal M1 radiation, which
appears as an exponential spike (LEMAR) at zero energy in the $\gamma$ strength
function of spherical nuclei. In deformed nuclei a Scissors Resonance appears
and LEMAR changes to damped magnetic rotation, which is interpreted as partial
restoration of coherence.
