arXiv subjects
Publications and source records attributed to A. F. R. de Toledo Piza.
We analyse a class of quantum dynamical processes which may lead to the hindering of the decay of a non-stationary state through appropriate entanglement with an additional two-level system. In this case the process can be considered as a module whose iteration is related to dynamical implementations of the so called quantum Zeno effect.
Using a collective potential derived on the basis of the Generator Coordinate Method with Skyrme interactions we obtain values for the compression modulus of 40Ca which are in good agreement with a recently obtained experimental value. Calculated values for the compression modulus for 16O are also given. The procedure involved in the derivation of the collective potential is briefly reviewed and discussed.
The collective excitation of hybrid atomic-molecular condensate are studied using variational method. The dipole response of the system is studied in detail. We found that the out-of-phase dipole oscillation frequency increasing slowly with the detuning parameter and is only few times large than the trap frequency making the experimental observation of this mode of vibration feasible.
We construct a three-dimensional analytically soluble model of the nonlinear effects in Coulomb excitation of multiphonon Giant Dipole Resonances (GDR) based on the SU(2,1) algebra. The full 3-dimensional model predicts further enhancement of the Double GDR (DGDR) cross sections at high bombarding energies. Enhancement factors for DGDR measured in thirteen different processes with various projectiles and targets at different bombarding energies are well reproduced with the same value of the nonlinearity parameter with the exception of the anomalous case of $^{136}$Xe which requires a larger value.
We discuss the origin of the difference between the harmonic value of the width of the multiphonon giant resonances and the smaller observed value. Analytical expressions are derived for both the effective width and the average cross-section. The contribution of the Brink-Axel mechanism in resolving the discrepancy is pointed out.
We propose a soluble model to incorporate the nonlinear effects in the transition probabilities of the multiphonon Giant Dipole Resonances based on the SU(1,1) algebra. Analytical expressions for the multi-phonon transition probabilities are derived. For reasonably small magnitude of nonlinearity $x\simeq 0.1-0.2$ enhancement factor for the Double Giant Resonance excitation probabilities and the cross sections %in relativistic ion collisions reaches values $1.3-2$ compatible with experimental data.
We consider the Parity Nonconserving (PNC) mixing in the ground state of exotic (halo) nuclei caused by the PNC weak interaction between outer neutron and nucleons within nuclear interior. For the nucleus $^{11}Be$ as an example of typical nucleus with neutron halo, we use analytical model for the external neutron wave functions to estimate the scale of the PNC mixing. The amplitude of the PNC mixing in halo state is found to be an order of magnitude bigger than that of typical PNC mixing between the ``normal'' nuclear states in nearby nuclei. The enhanced PNC mixing in halo cloud is proportional to the neutron weak PNC potential constant $g^W_n$ only.
Using many-body theory we develop a set of formally exact kinetic equations for inhomogeneous condensate and one-body observables. The method is illutrated for phi^4 field theory in 1+1 dimensions. These equations, when computed with the help of time-dependent projection technique, lead to a systematic mean-field expansion. The lowest and the higher order terms correspond to, respectively, the gaussian approximation and the dynamical correlation effect.
We investigate the physics of elementary excitations for the so called relativistic scalar plasma system. Following the standard many-body procedure we have obtained the RPA equations for this model by linearizing the TDHFB equations of motion around equilibrium and shown that these oscillation modes give one-meson and two-fermion state of the theory. The resulting equations have a closed solution, from which one can examinate the spectrum of excitation modes. In partticular, our results indicate existence of bound state for certain region of phase diagram.
We investigate ground-state and excitation spectrum of a system of non-relativistic bosons in one-dimension interacting through repulsive, two-body contact interactions in a self-consistent Gaussian mean-field approximation. The method consists in writing the variationally determined density operator as the most general Gaussian functional of the quantized field operators. There are mainly two advantages in working with one-dimension. First, the existence of an exact solution for the ground-state and excitation energies. Second, neither in the perturbative results nor in the Gaussian approximation itself we do not have to deal with the three-dimensional patologies of the contact interaction . So that this scheme provides a clear comparison between these three different results.
Isospin is included in the description of Coulomb excitation of multiple giant isovector dipole resonances. In the excitation of even-even nuclei, a relevant portion of the excitation strength is shown to be associated with 1+ two-phonon states, which tends to be hindered or completely supressed in calculations in which the isospin degree of freedom is not considered. We find that the excitation cross sections is strongly dependent on the ground state isospin.
A semiclassical description of multiple giant resonance excitation and decay that incorporates incoherent fluctuation contributions of the Brink-Axel type is developed. Numerical calculations show that the incoherent contributions are important at low to intermediate bombarding energies.
The dynamics of the composition of uniform Bose condensates involving two species capable of reciprocal interconversion is treated in terms of a collective quasi-spin model. This collective model quickly reduces to classical form towards the thermodynamic limit. Quantum solutions are easily obtained numerically short of this limit which give insight into the dynamically relevant correlation processes.
We propose a soluble model to incorporate the nonlinear effects in the transition probabilities of the multiphonon Giant Dipole Resonances based on the SU(1,1) algebra. Analytical expressions for the multi-phonon transition probabilities are derived. Enhancement of the Double Giant Resonance excitation probabilities in relativistic ion collisions scales as $(2 k +1)(2k)^{-1}$ for the degree of nonlinearity $(2k)^{-1}$ and is able to reach values $1.5-2$ compatible with experimental data. The enhancement factor is found to decrease with increasing bombarding energy. [KEYWORDS: Relativistic Heavy Ion Collisions,Double Giant Resonance]
We argue that a substantial increase in the cross section for Coulomb excitation in the region of the Double Giant Dipole Resonance should be expected from Coulomb excitation of excited states involved in the spreading of the one-phonon resonance, in a manifestation of the Brink-Axel phenomenon. This generates an additional fluctuating amplitude and a corresponding new term to be added incoherently to the usual cross-section. The appropriate extension of an applicable reaction calculation is considered in order to estimate this effect.
The decay theory of double giant resonances incorporating fluctuation contributions of the Brink-Axel type is developed. The gamma and neutron emission decay of Double Giant Dipole Resonances (DGDR) in 208Pb is discussed in connection with a recent measurement.
The dynamical effects of ground state correlations for excitation energies and transition strengths near the superfluid phase transition are studied in the soluble two level pairing model, in the context of the particle-particle self consistent Random Phase Approximation (SCRPA). Exact results are well reproduced across the transition region, beyond the collapse of the standard particle-particle Random Phase Approximation. The effects of two-body correlation in the SCRPA are displayed explicitly.
We show that two-atom correlation measurements of the type involved in a recent experimental study of the evolution of a mesoscopic superposition state prepared in a definite mode of a high-Q cavity can be used to determine the eigenvalues of the reduced density matrix of the field, provided the assumed dynamical conditions are actually fulfilled to experimental accuracy. These conditions involve i) a purely dispersive coupling of the field to the Rydberg atoms used to manipulate and to monitor the cavity field, and ii) the effective absence of correlations in the ground state of the system consisting of the cavity coupled to the ``reservoir'' which accounts for the decoherence and damping processes. A microscopic calculation at zero temperature is performed and compared to master equation results.