[en] Short note. 3 refs

[en] Two limiting cases of nuclear dynamics are analysed: the disappearing of giant collective motions in hot nuclei and the nuclear disassembly in violent heavy ion collisions. For collective vibration build on excited states we get a dramatic increase of the widths of hot Giant Dipole Resonances (GDR). As a consequence of the competition with particle evaporation we get a sharp quenching of giant photon emission. Pre-equilibrium effects on the GDR formation are also accounted for. At high temperature and low density the collective motions can become unstable leading to multifragmentation events in heavy ion collisions. We present a general procedure to identify instability regions and to get informations on the instability point. Some hints towards fully dynamical picture of multi-fragmentation processes are finally suggested. (author)

[en] Two limiting cases of nuclear dynamics are analysed in details: the disappearing of giant collective motions in hot nuclei and the nuclear disassembly in violent heavy ion collisions. It is shown that sharp transitions occur in the dynamical behaviour of finite nuclei. For collective vibration built on excited states we get a dramatic increase of the widths due to the enhancement of two body collisions with increasing temperature. The case of hot Giant Dipole Resonances (GDR) is discussed. As a consequence of the competition with neutron evaporation we get a sharp quenching of giant photon emission. Pre-equilibrium effects on the GDR formation are also accounted for. Limiting temperatures for the observation of GDR γ-decays are deduced for various nuclei ranging from 6.0 to 3.5 MeV with increasing mass number. A detailed study of the onset of a new multifragmentation mechanism in violent heavy ion collisions around 50 MeV/u beam energy for symmetric partners is performed. (author)

[en] We consider coherent atomic tunneling between two weakly coupled Bose-Einstein condensates at T=0 in a double-well trap. The condensate dynamics of the macroscopic amplitudes in the two wells is modeled by two Gross-Pitaevskii equations (GPE) coupled by a tunneling matrix element. Analytic elliptic function solutions are obtained for the time evolution of the inter-well fractional population imbalance z(t) (related to the condensate phase difference) of the Boson Josephson junction (BJJ). Surprisingly, the neutral-atom BJJ shows (non-sinusoidal generalizations of) effects seen in charged- electron superconductor Josephson junctions (SJJ). The BJJ elliptic-function behavior has a singular dependence on a GPE parameter ratio Λat a critical ratio Λ= Λ_c, beyond which a novel 'macroscopic quantum self-trapping' effect sets in with a non-zero time-averaged imbalance (z) ≠ 0. (Copyright (1998) World Scientific Publishing Co. Pte. Ltd)

[en] Extremely challenging new features of nuclear dynamics with relatively sharp transitions in nuclear stability have been observed. A new multifragmentation mechanism in heavy ion collisions in the range of 50 MeV per nucleon for symmetric beams is described. It is shown that the nuclear system enters a dynamical instabilities region and fragments are directly produced from the growing. 33 refs., 11 figs

[en] The effect of one- and two-body dissipation on the damping of giant dipole resonances (GDR) is studied in a semiclassical approach solving a Vlasov equation with a collision relaxation time. The latter is microscopically evaluated from the equilibration of a distorted momentum distribution in a kinetic approach. Temperature effects are introduced in the initial distribution function and in Pauli blocking rearrangement in the path to equilibration. Particle emission is also computed in the same microscopic picture. Without free parameters a good agreement with data is obtained for GDR's on the ground state if one include a large contribution from the Landau Damping. For collective vibrations built on excited state we get a dramatic increase of the widths due to enhancement of nucleon-nucleon (NN) collisions. The saturation observed in some experiments is explained as due to the competition of particle evaporation which cools down the system. The transition to first sound modes seems to be ruled out for the persistence of long nucleon mean free paths at relatively high temperatures. For dissipative heavy ion collisions at medium energy we study equilibrium and non-equilibrium features of complex fragment production. We consider first statistical sequential emission from equilibrated primary sources formed in the entrance channel dynamics. The inputs of cascade calculations are derived from a suitable clustering procedure at some carefully fixed equilibration time. Large pre-equilibrium dissipation is revealed. Applications are shown to the reactions ¹³⁹ La + ²⁷ Al and ¹³⁹ La + ⁶³ Cu at 55 MeV/u beam energies. Absolute comparisons with experiments show a quite good overall agreement. Some discrepancies revealed for high multiplicity events and when a larger excitation energy in available, appear as a signature of the onset of a direct multifragmentation mechanism. A microscopic study of the nuclear matter phase diagram in the participant zone clearly shows the relation of this multifragment emission to dynamical instabilities. (Author)

[en] We consider coherent atomic tunneling between two weakly coupled Bose-Einstein condensates (BEC) at T = 0 in (possibly asymmetric) double-well trap. The condensate dynamics of the macroscopic amplitudes in the two wells is modeled by two Gross-Pitaevskii equations (GPE) coupled by a tunneling matrix element. The evolution of the inter-well fractional population imbalance (related to the condensate phase difference) is obtained in terms of elliptic functions, generalizing well-known Josephson effects such as the 'ac' effect, the 'plasma oscillations', and the resonant Shapiro effect, to the nonsiusoidal regimes. We also present exact solutions for a novel 'macroscopic quantum self-trapping' effect arising from nonlinear atomic self-interaction in the GPE. The coherent BEC tunneling signatures are obtained in terms of the oscillations periods and the Fourier spectrum of the imbalance oscillations, as a function of the initial values of GPE parameters. Experimental procedures are suggested to make contact with theoretical predictions. (author). 44 refs, 8 figs

[en] We analyze the response of condensed trapped atoms to external driving magnetic fields. Solving the time-dependent Gross-Pitaewskii equation within a new, accurate algorithm, we found frequencies and relaxation times of collective oscillations in excellent agreement with the experimental results. Using simple scaling arguments we make quantitative predictions for future experiments. copyright 1997 The American Physical Society

[en] We study the quantum and the mean-field Gross-Pitaevskii (GP) dynamics of a Bose-Einstein condensate gas confined in a toroidal trap. According to GP, if the interatomic interaction is attractive, the rotational states of the system can be dynamically stable or unstable depending on the strength of the mean-field energy. The full quantum analysis, however, reveals that the condensate is always unstable. Quantum fluctuations are particularly important close to the GP stability borderline, even for systems with a relatively large number of condensate atoms

[en] The quadrupole excitations in nucleus ¹¹Li have been studied within the nuclear Vlasov equation approach. The classical phase space distribution function is given by the test particle propagation method. A soft mode of quadrupole excitation located around 2 MeV is found. The total percentage of E2 energy-weighted sum rule exhausted in this soft resonance region is very sensitive to the extension of neutron halo in nucleus ¹¹Li. The results compared with other recent calculations are analyzed