Among the numerous different ways to excite many-body and other complex quantum systems, mechanisms are often found which are clearly distinguished by a simple, typically semiclassical interpretation. In nuclei, these are the collective excitations in which all or large groups of particles move coherently. They often act as "doorways" to other excitations of single-particle character. Examples for and the limitations of the doorway mechanism are discussed. Recent results show that superscars in the barrier billiard serve as perfect object to shed light on aspects of the doorway mechanism which are not directly accessible in traditional quantum systems. To this end, two new statistical observables are employed. Some open questions are addressed.
Tensor effects in the dipole excitation of neutron-rich ^{68}Ni nucleus are investigated in the framework of Skyrme-Hartree-Fock plus random phase approximation at finite temperature. We calculate isovector giant and pygmy dipole strengths with finite temperature random phase approximation by using different tensor correlations. The effect of both tensor and finite temperature on the giant dipole resonance-pygmy dipole resonance energy region is analysed. Pygmy dipole resonance calculations with different proton-neutron tensor coupling constants are also compared with the experimental results.
The validity of the small damping approximation (SDA) for the quasi-classical description of the averaged properties of nuclei at high temperatures is studied within the framework of collisional kinetic theory. The isoscalar collective quadrupole vibrations in hot nuclei are considered. We show that the extension of the SDA, by accounting for the damping of the distribution function δ f in the collision integral, reduces the rate of variation of distortion effects of the Fermi surface with temperature. The damping of the δ f in the collision integral increases the collisional width of the giant quadrupole resonance (GQR) significantly for small enough values of the relaxation time. The temperature dependence of the eigenenergy of the GQR becomes much weaker than in the corresponding SDA case.
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