Collective excitations in superconductors

The excitations connected with quantum condensation of a fermion gas are calculated quantitatively along the lines of a previous paper. We find exact agreement with earlier results: excitations of the phonon type in the absence of Coulomb interaction, strong indications for their nonexistence in the presence of this interaction.     

Collective excitations of gluons

Three gauge invariant antisymmetric tensor fields are introduced in the nonabelian gauge theories. They are certain non-linear combinations of the conjugate field tensor and they obey O(3) algebra. An effective chiral lagrangian for these fields is derived. It describes 3 vector and 3 axial mesons with vacuum quantum numbers. The masses are generated by spontaneous restoration of Lorentz invariance.     

Collective excitations in semiconductor superlattices

Collective excitations in semiconductor superlattices are studied beyond tbe random-phase approximation (RPA). The Singwi, Tosi, Land and Sjölander (STLS) theory, which accounts for exchange and short-range correlations effects through an effective potential depending on the structurc factor, is generalized to the layered electron system described by the model of Visscher and Falicov. The exact numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The plasmon dispersion curves are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the planes for GaAs/AlGaAs semiconductor superlattices. For comparison, the RPA and the Hubbard approximation are also considered.     

Collective excitations of the polaron-gas

The dispersion of the collective excitations of a polaron-gas is calculated. It is found that the interaction between the L.O. phonons and the plasmons induces a relatively strong wave vector dependence in the ω⁺, ω^- modes. This dispersion is primarily a consequence of the interaction of the L.O. phonons with the resonant plasmon branch in the pair excitation spectrum.     

Collective excitations in electron-hole bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.     

Collective excitations in exciton crystal

Crystal phase of indirect excitons formed by spatially separated electrons and holes in coupled quantum wells is analyzed. The collective mode spectrum of the exciton crystal at zero and nonzero agnetic fields is found. The spectrum consists of two optical and two acoustical modes (transverse and longitudinal in each case). We also study changes of the dipole crystal collective excitations at the transition exciton crystal-electron-hole plasma.     

Collective excitations of nuclear matter

It has been shown that in a renormalizable gauge-invariant field-theory model, besides the usual factorizable contribution to the parity-violating vector-meson-exchange potential there is a nonseparable contribution which is equally important. Both contributions have been calculated in the model based on the Bethe-Salpeter equation which describes vector mesons as bound quark-antiquark states. Complete expressions for parity-violating nuclear potentials have been given for the Cabibbo and the Weinberg-Salam models, including the asymptotically-free field-theory version of the latter, which employs coloured quarks and vector gluons.     

Collective excitations in145Ba

High-spin levels in¹²⁵Ba have been produced in the¹¹⁶Sn(¹² C, 3n) reaction and studied by in-beam spectroscopic methods. Two strongly populated band structures are observed. The odd-parity one is based on a 7/2^? state and can be explained as the result of the coupling between anh _11/2 neutron-hole and a prolate type triaxial core. The even-parity band, built on a 7/2⁺ state, corresponds to collective excitations associated with a neutron-hole in theg _7/2 shell. Comparisons with heavier odd-A Ba isotopes and discussions are made in the frame work of the triaxial core model. Nuclear Reactions¹¹⁶Sn(¹²C, 3nγ), E=45–55 MeV; measuredσ(E;E _γ,θ),γγ-coinc,γ-γ delay. Enriched target, Ge(Li) detectors.¹²⁵Ba deduced levels,J,π,γ-mixing.     

Collective excitations in molecular complexes

Green's functions are used to derive an equation for the energies of collective excitations in molecular complexes. It is found that the collective branch contains a solution of exciton type. An expression is derived for the energies of the exciton excitations.     

The rotational quasi-continuum in specific configurations: high-K states and superdeformation

The analysis of the fluctuations of the counts of unresolved γ-γ energy spectra can extend our knowledge of the rotational motion at high angular momentum and excitation energy above the yrast band. Detailed studies of the rotational quasi-continuum built on specific intrinsic nuclear configurations are here discussed. First, the validity of the K-selection rules in the quasi-continuum region is investigated in the case of the normal deformed nucleus ¹⁶³Er. The γ cascades feeding into low-K and high-K bands are analyzed studying variance and covariance of the spectrum fluctuations. Low-K bands are found to be fed by a much larger effective number of cascades than high-K bands, while the covariance between pairs of gated spectra shows that the cascades feeding into low-K bands are different from those feeding into high-K bands. These findings suggest a persistence of K-selection rules in the region of excitation energy and angular momentum probed by the rotational decay of ¹⁶³Er. As a second case, the existence of a rotational quasi-continuum in the super deformed well of ¹⁴³Eu is discussed. An intense bump of rotationally correlated transitions is observed to develop as function of fold in the high spin region of gated spectra. Fold distribution, angular anisotropies, moment of inertia and life time show that the bump consists of fully damped super deformed transitions. This gives experimental evidence of damped rotational motion up to several MeV excitation energy above the super deformed yrast line.     

Collective excitations in small carbon cluster

We apply the time-dependent local density approximation to the small carbon clusters. The carbon clusters of chain and ring shapes are found to show strong π-electron transitions. An interpretation is given for them as the one-dimensional plasmon excitation. Presented by K. Yabana at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Collective excitations in the anharmonic crystal

We consider a lattice model of an anharmonic crystal and compute the isothermal (χ_T) and adiabatics (χ_S) susceptibilities for the lattice displacement below T_c. A detailed analysis is given in terms of the dimension of the lattice and a decay parameter of the harmonic force. It turns out that there is a phase-space region for which 0 < χ_T/χ_S < 1, indicating breaking of ergodicity, and a region for which χ_T is finite, indicating the presence of a plasmon frequency due to the anharmonicity.