Study of a fast collective mode in deep inelastic reactions: Quantal fluctuations

We have measured for the 430 MeV⁸⁶Kr+^92.98Mo systems the triple differential cross sectiond ³ σ/dEdAdZ (E=kinetic energy,A=mass andZ=atomic number) of the reaction products in the angular range (θ=25, 45°). We focussed attention on a fast collective mode in deep inelastic reactions: the neutron excess or charge equilibration. This mode is shown to relax very quickly to equilibrium within a time scale of the order of 10^?22s. It is shown that this collective degree of freedom exhibit a quantal behavior which can be seen in the observation of quantal fluctuations. The experimental results are discussed in terms of a simple equilibrium model. The giant dipole resonance of the composite system could be closely connected to this neutron excess mode.     

Quantal fluctuations of the magnetic field in fermion system

The effects of quantal fluctuations of the magnetic field in a system of structureless interacting fermions generating self-organized macroscopic and periodic magnetic structures are analysed. The analysis is made by using the functional integration methods.The autor is grateful to Professor M. Noga for many enlightening conversations and useful advices.     

Very-short-wavelength collective modes in fluids

The existence of very-short-wavelength collective modes in fluids is discussed. These collective modes are the extensions of the five hydrodynamic (heat, sound, viscous) modes to wavelengths of the order of the mean free path in a gas or to a fraction of the molecular size in a liquid. They are computed here explicitly on the basis of a model kinetic equation for a hard sphere fluid. At low densities all five modes are increasingly damped with decreasing wavelength till each ceases to exist at a cutoff wavelength. At high densities the extended heat mode softens very appreciably for wavelengths of the order of the size of the particles and becomes a diffusion-like mode that persists till much shorter wavelengths than the other modes. Except for the shortest wavelengths these collective modes and in particular the heat mode dominate the dynamical structure factorS(k, ) for all densities. The agreement of the theory with experimentalS(k, ) of liquid Ar seems to imply that very-short-wavelength collective modes also occur in real fluids.     

Coherent collective modes in catalytic RNA

We establish the existence ofФ ⁴-solitons travelling along the backbone of an RNA double stranded helix. Taking advantage of the catalytic activity of RNA in proton transfer events, we device a probe for such coherent solitary-wave excitations: We prove that the density of domain walls is increased by the same phonon excitations which would enhance proton exchange. The rationale behind our conclusion is that the uracil bases contained within the domain wall are tautomerized.     

Longitudinal collective modes in liquid water

Summary Results of extensive computer simulations performed in liquid water are reported in order to clarify the main features of dynamical density correlations in this system. Puzzling aspects, such as the anomalous sound wave propagation at finite wave vectors, are related to the peculiar structural aspects of this hydrogen-bonded liquid. Additional features (such as the evolution of the relevant memory functions at increasing wave vectors and the emergency of a second mode in the longitudinal current spectra) are also discussed in some detail. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Damping of coupled phonon-plasmon modes

The effect of free carriers on the dispersion and damping of coupled phonon-plasmon modes is considered in the long-wave approximation. The electron and phonon scattering rate, as well as Landau damping, is taken into account.     

Damping of nuclear collective and single-particle motion

The collective motion of the nuclear system is studied. In the independent-particle model, the motion is completely reversible. The neglected residual interactions couple the ph states to more complicated states. This coupling is taken into account by the optical model potential assuming independent decay of particle and hole states. Irreversibility is thereby introduced and damping of collective motion described in terms of the widths of the ph states. The validity of the assumption of independent decay is discussed. It is argued that spreading widths to low-frequency collective states are not part of the optical model, and do not contribute to damping of collective motion.     

Pairing collective modes in superfluid nuclei: a semiclassical approach

Collective pairing excitations in superfluid nuclei are studied by using the semiclassical time-dependent Hartree-Fock-Bogoliubov equations of motion in a linear approximation. It is found that the strength function associated with the semiclassical anomalous density response function has a resonance structure with a sharp peak around double energy gap that displays the monopole pairing collective mode.     

Transverse collective modes in liquid binary alloys

Transverse current correlations in binary liquid alloys are investigated by molecular dynamics simulation. The study includes several Li-Mg, Li-Na and Li-Pb alloys. The characteristics of both shear and transverse optic-like modes are discussed. The former modes are associated with the number density fluctuations whereas the latter are related to the concentration fluctuations. Special attention is paid to the dependence of the results on the mass ratio and composition of the alloy.     

Pinning frequencies of the collective modes in alpha-uranium

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline alpha-uranium. In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature ThetaD = 265 K. In the polycrystalline sample the heat capacity curve is thermally broadened (ThetaD = 184 K), but no excess heat capacity was observed. The excess heat capacity Cphi (taken as the difference between the single-crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency V0 for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal.     

Damping of high-lying single-particle modes in heavy nuclei

The recent experimental and theoretical results on the damping of high-lying single-particle modes in heavy nuclei are reviewed. In one-nucleon transfer reactions these states manifest themselves as broad “resonance”-like structures superimposed on a large continuum. The advantages and the limitations of the transfer reaction approach will be presented using the results from neutron and proton pick-up and stripping reactions. The problem raised by the subtraction of the underlying background, the assumptions made to describe the reaction process and the method used to extract the strength distributions are presented. The existing empirical systematics is summarized for nuclei ranging from ⁹⁰Zr to ²⁰⁸Pb.The theoretical approaches used to explain the damping of the high-lying single-particle modes are based on the coupling between collective and single-particle degrees of freedom. In a first step the bare single-particle mode is spread over several doorway collective states due to the interaction with surface vibrations. In a second step the doorway states spread their strengths over many other degrees of freedom. These two steps of the damping mechanism are discussed in detail within the framework of the quasiparticle-phonon nuclear model. A large-scale comparison between the measured and calculated average energies, spreading widths and spectroscopic strengths of the high-lying single-particle (hole) states in heavy nuclei is presented. The systematic features of the damping (energy, angular momentum and isotopic dependence) are discussed. Recent advances of the experimental approaches, such as the γ-decay of the high-lying states or the use of heavy-ion transfer reactions at intermediate energies, are outlined.The detailed study of the damping mechanism of high-lying single-particle modes reveals new features and leads us to a new field in nuclear structure: “the spectroscopy of inner and outer subshells”.     

Observation of collective modes of ultracold plasmas

Applying a radio-frequency electric field to an expanding ultracold neutral plasma leads to the observation of as many as six peaks in the emission of electrons from the plasma. These are identified as collective modes of the plasma and are in qualitative agreement with a model of Tonks-Dattner resonances, electron sound waves propagating in a finite-sized, inhomogeneous plasma. Such modes may provide an accurate method to determine the time-dependent electron temperature.     

Damping of nuclear collective motion in the extended mean-field theory

The dissipation mechanism in slow nuclear collective motion is studied in the frame of the extended mean-field theory. The collective motion is treated explicitly by employing a travelling single-particle representation in the semi-classical approximation. The rate of change of the collective kinetic energy is determined by: (i) one-body dissipation, which reflects uncorrelated particle-hole excitations as a result of the collisions of particles with the mean field, (ii) two-body dissipation, which consists of simultaneous 2 particle-2 hole excitations via direct coupling of the residual two-body interactions, and (iii) potential dissipation due to the redistribution of the single-particle energies as a result of the random two-body collisions. In contrast to the first two processes the potential dissipation exhibits memory effects due to the large values of the local equilibration times.     

Quadrupole collective modes in trapped finite-temperature Bose-Einstein condensates

Finite-temperature simulations are used to study quadrupole excitations of a trapped Bose-Einstein condensate. We focus specifically on the m = 0 mode, where a long-standing theoretical problem is to account for an anomalous variation of the mode frequency with temperature. We explain this behavior in terms of the excitation of two separate modes, corresponding to the coupled motion of the condensate and the thermal cloud. The relative amplitude of the modes depends sensitively on the temperature and on the frequency of the harmonic drive used to excite them. Good agreement with experiment is found for appropriate drive frequencies.