arXiv subjects

Klaus Morawetz

Publications and source records attributed to Klaus Morawetz.

69 records · Page 4Linked to original sources

arXiv1999

Damping of IVGDR - Fermi-liquid or Fermi-gas ?

Collisional relaxation rates of collective modes in nuclei are calculated using the Levinson equation for the reduced density matrix with a memory dependent collision term. Linearizing the collision integral two contribution have to be distinguished, the one from the quasiparticle energy and the one from occupation factors. The first one yields the known Landau formula of zero sound damping and the second one leads to the Fermi gas model of Ref.1 with the additional factor 3 in front of the frequencies. Adding both contribution we obtain a final relaxation rate for the Fermi liquid model. Calculations of the temperature dependence of the damping rates and of the shape evolution of IVGDR are in good agreement with the experiment and show only minor differences between both models.

nucl-th↗

arXiv1999

ISGDR- bulk or surface mode ?

A polarization and response function for finite systems and temperatures is derived from linearizing the Vlasov equation. Besides the Lindhard response function in local density approximation we obtain an additional contribution due to the surface. This formula is applied to isoscalar giant resonances which we consider as next harmonics to the basic dipole mode. The spurious center of mass motion is subtracted within the polarization. The results based on simple Fermi liquid considerations are in reasonable agreement with experimental data.

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arXiv1999

Chaotic scattering on surfaces and collisional damping of collective modes

The damping of hot giant dipole resonances is investigated. The contribution of surface scattering is compared with the contribution from interparticle collisions. A unified response function is presented which includes surface damping as well as collisional damping. The surface damping enters the response via the Lyapunov exponent and the collisional damping via the relaxation time. The former is calculated for different shape deformations of quadrupole and octupole type. The surface as well as the collisional contribution each reproduce almost the experimental value, therefore we propose a proper weighting between both contributions related to their relative occurrence due to collision frequencies between particles and of particles with the surface. We find that for low and high temperatures the collisional contribution dominates whereas the surface damping is dominant around the temperatures $\sqrt{3}/2π$ of the centroid energy.

astro-ph↗

arXiv1999

Giant Octupole Resonance Simulation

Using a pseudo-particle technique we simulate large-amplitude isoscalar giant octupole excitations in a finite nuclear system. Dependent on the initial conditions we observe either clear octupole modes or over-damped octupole modes which decay immediately into quadrupole ones. This shows clearly a behavior beyond linear response. We propose that octupole modes might be observed in central collisions of heavy ions.

nucl-th↗

arXiv1999

Space-time versus particle-hole symmetry of collisions in a non-local kinetic equation for dense Fermi systems

Binary collisions in Fermi systems obey two fundamental symmetries corresponding to the space and time inversion and to the interchange of particles and holes. We show that beyond the local and instant approximation of scattering-in and -out integrals of a kinetic equation, only one of symmetries can be explicit while the other has to be covered by a constraint. This constraint, derived from the optical theorem, allows one to convert at need an explicit particle-hole symmetric form to the space-time symmetric form and vice versa. We implement this constraint to heavy ion reactions, where simulation algorithms require the space-time symmetry while former theories offer kinetic equations with the explicit particle-hole symmetry.

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arXiv1999

Duration and non-locality of a nucleon-nucleon collision

For a set of realistic nucleon-nucleon potentials we evaluate microscopic parameters of binary collisions: a time duration of the scattering state, a mean distance and a rotation of nucleons during a collision. These parameters enter the kinetic equation as non-instantaneous and non-local corrections of the scattering integral, i.e., they can be experimentally tested. Being proportional to off-shell derivatives of the scattering T-matrix, non-instantaneous and non-local corrections make it possible to compare the off-shell behavior of different potentials in a vicinity of the energy shell. The Bonn one-Boson-exchange (A-C) and Paris potentials are found to yield very close results, while the separable Paris potential differs.

nucl-th↗

arXiv1999

Virial corrections to simulations of heavy ion reactions

Within QMD simulations we demonstrate the effect of virial corrections on heavy ion reactions. Unlike in standard codes, the binary collisions are treated as non-local so that the contribution of the collision flux to the reaction dynamics is covered. A comparison with standard QMD simulations shows that the virial corrections lead to a broader proton distribution bringing theoretical spectra closer towards experimental values. Complementary BUU simulations reveal that the non-locality enhances the collision rate in the early stage of the reaction. It suggests that the broader distribution appears due to an enhanced pre-equilibrium emission of particles.

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arXiv1999

Quantum response of finite Fermi systems and the relation of Lyapunov exponent to transport coefficients

Within the frame of kinetic theory a response function is derived for finite Fermi systems which includes dissipation in relaxation time approximation and a contribution from additional chaotic processes characterized by the largest Lyapunov exponent. A generalized local density approximation is presented including the effect of many particle relaxation and the additional chaotic scattering. For small Lyapunov exponents relative to the product of wave vector and Fermi velocity in the system, the largest Lyapunov exponent modifies the response in the same way as the relaxation time. Therefore the transport coefficients can be connected with the largest positive Lyapunov exponent in the same way as known from the transport theory in relaxation time approximation.

physics.atom-ph↗

arXiv1998

Non-instant collisions and two concepts of quasiparticle

The kinetic theory recently implemented in heavy ion reactions combines a non-local and non-instant picture of binary collisions with quasiparticle features. We show that the non-instant description is compatible with the spectral concept of quasiparticles while the commonly used variational concept is consistent only with instant collisions. The rearrangement energy, by which the variational concept surpasses the spectral one, is shown to be covered by a medium effect on non-instant collisions.

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arXiv1998

Nonlocal Kinetic Equation and Simulations of Heavy Ion Reactions

A kinetic equation which combines the quasiparticle drift of Landau's equation with a dissipation governed by a nonlocal and noninstantaneous scattering integral in the spirit of Enskog corrections is discussed. Numerical values of the off-shell contribution to the Wigner distribution, of the collision duration and of the collision nonlocality are presented for different realistic potentials. On preliminary results we show that simulations of quantum molecular dynamics extended by the nonlocal treatment of collisions leads to a broader proton distribution bringing the theoretical spectra closer towards the experimental values than the local approach.

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arXiv1998

Kinetic equation consistent with the equation of state of nuclear matter

A kinetic equation which combines the quasiparticle drift of Landau's equation with a dissipation governed by a nonlocal and noninstant scattering integral in the spirit of Snider's equation for gases is derived. Consequent balance equations for the density, momentum and energy include quasiparticle contributions and the second order quantum virial corrections. The medium effects on binary collisions are shown to mediate the latent heat, i.e., an energy conversion between correlation and thermal energy. An implementation to heavy ion collisions is discussed.

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arXiv1998

Formation of binary correlations in plasma

The formation of binary correlations in plasma is studied from the quantum kinetic equation. It is shown that this formation is much faster than dissipation due to collisions, in hot (dense) plasma the correlations form on the timescale of inverse plasma frequency (Fermi energy). This hierarchy of characteristic times is used to derive analytical formulae for time dependency of the potential energy of binary interactions which measures the extent of correlations. We discuss the dynamical formation of screening and compare with the statical screened result.Comparisons are made with molecular dynamic simulations. In the low temperature limit we find an analytical expression for the formation of correlation which is general for any binary interaction. It can be applied in nuclear situations as well as dense metals.

cond-mat.str-el↗

arXiv1998

Quasiparticle transport equation with collision delay. I. Phenomenological approach

For a system of non-interacting electrons scattered by resonant levels of neutral impurities, we show that virial and quasiparticle corrections have nearly equal magnitudes. We propose a modification of the Boltzmann equation that includes quasiparticle and virial corrections and discuss their interplay on a dielectric function.

cond-mat.dis-nn↗

arXiv1998

Quasiparticle transport equation with collision delay. II. Microscopic Theory

For a system of non-interacting electrons scattered by neutral impurities, we derive a modified Boltzmann equation that includes quasiparticle and virial corrections. We start from quasiclassical transport equation for non-equilibrium Green's functions and apply limit of small scattering rates. Resulting transport equation for quasiparticles has gradient corrections to scattering integrals. These gradient corrections are rearranged into a form characteristic for virial corrections.

cond-mat.dis-nn↗

arXiv1993

In-medium relativistic kinetic theory and nucleon-meson systems

Within the $σ-ω$ model of coupled nucleon-meson systems, a generalized relativistic Lenard--Balescu--equation is presented resulting from a relativistic random phase approximation (RRPA). This provides a systematic derivation of relativistic transport equations in the frame of nonequilibrium Green's function technique including medium effects as well as flucuation effects. It contains all possible processes due to one meson exchange and special attention is kept to the off--shell character of the particles. As a new feature of many particle effects, processes are possible which can be interpreted as particle creation and annihilation due to in-medium one meson exchange. In-medium cross sections are obtained from the generalized derivation of collision integrals, which possess complete crossing symmetries.

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