Starting from a nonmarkovian conserving relaxation time approximation for collisions we derive coupled dispersion relations for asymmetric nuclear matter. The isovector and isoscalar modes are coupled due to asymmetric nuclear meanfield acting on neutrons and protons differently. A further coupling is observed by collisional correlations. The latter one leads to the appearance of a new soft mode besides isoscalar and isovector modes in the system. We suggest that this mode might be observable in asymmetric systems. This soft mode approaches the isovector mode for high temperatures. At the same time the isovector mode remains finite and approaches a constant value at higher temperatures showing a transition from zero sound like damping to first sound. The damping of the new soft mode is first sound like at all temperatures.The investigation of collective excitations in asymmetric nuclear matter is of current interest for experiments with nuclei far from β-stability, [1] and citations therein. We consider here a Fermi gas model consisting of a number of different species (neutrons, protons, etc) interacting with the own specie and with other ones. The interaction between different sorts of particles is important to consider if we want to include friction between different streams of isospin components. Especially the isospin current may not be conserved by this way. We neglect explicitly shell effects and concentrate only on bulk matter properties. Let us start with a set of coupled quantum kinetic equations for the reduced density operator ρ a for the specie awhere E = P 2 /2m denotes the kinetic energy operator and U the mean field operator and the external field which is assumed to be a nonlinear function of the density. We have approximated the collision integral by a non-Markovian relaxation time [8]. The memory effects condensed in the frequency dependent relaxation time turned out to be necessary to reproduce damping of zero sound [2,3]. It accounts for the fact that during a two particle collision a collective mode can couple to the scattering process. Consequently, the dynamical relaxation time represents the physical content of a hidden three particle process and is equivalent to the memory effects.We have further assumed the relaxation with respect to the local equilibriumρ b of any specie in the system. The relaxation of the actual distribution of specie a is driven by the local equilibrium of all the other components. The cross coupling is realized by nondiagonal relaxation times τ ab . We specify the local equilibrium by a small deviation of the chemical potential of specie a [4] compared with the global equilibrium (2) with < k|E|k ′ >= ǫ(k). The equilibrium distributions < k|ρ 0 a |k ′ >= f a (k)δ kk ′ are the corresponding Fermi functions with chemical potential µ a and the normalization to density n a = 2 dp (2π) 3 f a (p). The local equilibrium specified by δµ a is determined if we impose the density balance to be fulfilled separately for each specie current J a which reads in Wigner coordinat...

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The damping rate of hot giant dipole resonances (GDR) is investigated. Besides Landau damping we consider collisions and density fluctuations as contributions to the damping of GDR. Within the non-equilibrium Green's function method we derive a non-Markovian kinetic equation. The linearization of the latter one leads to complex dispersion relations. The complex solution provides the centroid energy and the damping width of giant resonances. The experimental damping widths are the full width half maximum (FWHM) and can be reproduced by the full width of the structure function. Within simple finite size scaling we give a relation between the minimal interaction strength which is required for a collective oscillation and the clustersize. We investigate the damping of giant dipole resonances within a Skyrme type of interaction. Different collision integrals are compared with each other in order to incorporate correlations. The inclusion of a conserving relaxation time approximation allows to find the T 2 -dependence of the damping rate with a temperature known from the Fermi-liquid theory. However, memory effects turn out to be essential for a proper treatment of the damping of collective modes. We derive a Landau like formula for the one-particle relaxation time similar to the damping of zero sound. 21.30.Fe,21.60.Ev, 24.30.Cz, 24.60.Ky

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