Collective modes in asymmetric nuclei
  The collective motion of a finite nuclear system is investigated by numerical simulation and by linear response theory. Using a pseudo-particle simulation technique we analyze the giant resonances with a multipole decomposition scheme. We examine the energy and the damping of different giant collective modes and obtain the dependence of these quantities on the proton-neutron ratio. ...(abbreviated) Alternatively the giant collective modes in asymmetric nuclear matter are investigated within linear response theory including the collisional correlations via a dynamic relaxation time approximation. For a multicomponent system we derive a coupled dispersion relation and show that two sources of coupling appear: (i) a coupling of isoscalar and isovector modes due to the action of different mean-fields and (ii) an explicit new coupling in asymmetric matter due to collisional interaction. We show that the latter is responsible for a new mode arising besides isovector and isoscalar modes...(abbreviated) Collective motion beyond the linear regime is demonstrated for the example of large-amplitude isoscalar giant octupole excitations in finite nuclear systems. Depending on the initial conditions we observe either clear octupole modes or over-damped octupole modes which decay immediately into quadrupole ones. This dependence on initial correlations represents a behaviour beyond linear response. 
Klaus Morawetz, Uwe Fuhrmann, Rainer Walke