Time-dependent Relativistic Mean-field Theory Research Articles

The time-dependent relativistic mean-field theory and the random phase approximation

The Relativistic Random Phase Approximation (RRPA) is derived from the Time-Dependent Relativistic Mean-Field (TD RMF) theory in the limit of small amplitude oscillations. In the no-sea approximation of the RMF theory, the RRPA configuration space includes not only the usual particle–hole ph-states, but also αh-configurations, i.e. pairs formed from occupied states in the Fermi sea and empty negative-energy states in the Dirac sea. The contribution of the negative-energy states to the RRPA matrices is examined in a schematic model, and the large effect of Dirac-sea states on isoscalar strength distributions is illustrated for the giant monopole resonance in 116Sn. It is shown that, because the matrix elements of the time-like component of the vector-meson fields which couple the αh-configurations with the ph-configurations are strongly reduced with respect to the corresponding matrix elements of the isoscalar scalar meson field, the inclusion of states with unperturbed energies more than 1.2 GeV below the Fermi energy has a pronounced effect on giant resonances with excitation energies in the MeV region. The influence of nuclear magnetism, i.e. the effect of the spatial components of the vector fields is examined, and the difference between the nonrelativistic and relativistic RPA predictions for the nuclear matter compression modulus is explained.

Regular and chaotic dynamics in time-dependent relativistic mean-field theory

Isoscalar and isovector monopole oscillations that correspond to giant resonances in spherical nuclei are described in the framework of time-dependent relativistic mean-field theory. Time-dependent and self-consistent calculations that reproduce experimental data on monopole resonances in 208Pb, show that the motion of the collective coordinate is regular for isoscalar oscillations, and that it becomes chaotic when initial conditions correspond to the isovector mode. Regular collective dynamics coexists with chaotic oscillations on the microscopic level. Time histories, Fourier spectra, state-space plots, Poincare sections, autocorrelation functions, and Lyapunov exponents are used to characterize the nonlinear system and to identify chaotic oscillations. Analogous considerations apply to higher multipolarities.

Double giant resonances in time-dependent relativistic mean-field theory

Abstract Collective vibrations in spherical nuclei are described in the framework of time-dependent relativistic mean-field theory (RMFT). Isoscalar quadrupole and isovector dipole oscillations that correspond to giant resonances are studied, and possible excitations of higher modes are investigated. We find evidence for modes which can be interpreted as double resonances. In a quantized RMFT they correspond to two-phonon states.

Relativistic mean-field description of small- and large-amplitude collective motion in nuclei

The dynamics of collective motion in nuclei is described in the framework of time-dependent relativistic mean-field theory. For a given set of initial conditions (stationary solution for the ground state, initial velocities of the proton and neutron densities, initial deformations), the model describes the time evolution of the nuclear system. In the limit of small-amplitude motion we investigate isovector dipole, isoscalar quadrupole, and isovector quadrupole oscillations. Model calculations are performed for the nuclei 16O, 40Ca, 48Ca and compared with experimental data on energies and widths of giant resonances. For 48Ca we also investigate the dynamics of large-amplitude isovector dipole motion. With increasing excitation energy the motion becomes strongly anharmonic and clusters of particles evaporate from the surface of the nucleus. At the highest energies we find a rapid dissociation of the nucleus in proton and neutron clusters.

Dynamics of Coulomb dissociation in relativistic mean field theory

Time-dependent relativistic mean field theory is used to describe the dynamics of Coulomb excitations of nuclei. As an example we consider the nucleus 16O. At small incident energies of the projectile we observe isovector dipole oscillations that correspond to the giant dipole resonance. With increasing excitation energy, the motion becomes increasingly anharmonic, and finally a full dissociation of 16O into proton and neutron clusters is observed.

Coulomb excitations of a nuclear slab in relativistic mean-field theory

Time-dependent relativistic mean-field theory is used to describe the dynamic behaviour of a nuclear slab excited at time zero by a rapid Coulomb push. At small incident energies of the projectile, we observe oscillations of small amplitude with isovector character. With increasing excitation energy, the motion becomes increasingly anharmonic, and parts of the nuclear material are ejected. At the highest energies, we find a rapid dissociation between the proton and neutron part of the slab. It turns out that the character of the oscillations is strongly influenced by the strength of the ρ-meson coupling. For vanishing coupling, we observe two modes corresponding in nature to the Goldhaber-Teller mode and the Steinwedel-Jensen mode of the giant dipole resonance in realistic nuclei. At larger ρ-meson coupling, the Goldhaber-Teller mode is more and more suppressed, and one is left with one strongly damped mode of the Steinwedel-Jensen type.

Relativistic description of Coulomb dissociation. A one-dimensional model

Time-dependent relativistic mean field theory is used to describe the dynamics of a one-dimensional nuclear slab after an electromagnetic excitation. Depending on the strength of the excitation we observe harmonic oscillations, evaporation of parts of the slab and full Coulomb dissociation.