Critical phenomena in open quantum systems and collective excitations

In quantum systems at high level density, the states of the system are mixed strongly via the continuum of decay channels. This mixing creates states with large external collectivity if the level density reaches some critical value. Besides this external collectivity, the states may have some internal collectivity created by (internal) residual interaction in the corresponding closed system. The cross section pattern at high level density is determined by the interplay of internal and external collectivity. Presented at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.