Retardation effects: Localized excitations of a soliton in a model of a one-dimensional electron-phonon (Peierls) system

A set of localized mixed electron-phonon excitations has been found in the “quantum” regime M < K²/2C(g²+2KB) = M_c. We suggest that the spectrum is related to possible bound states of ions in the Coulomb field of the charge soliton for sufficiently light ions.     

Intensitive dependence of luminescence and photoconductivity for the electron-electron,electron-exciton and exciton-exciton interactions

Analytical solutions of the equations of kinetics were found for stationary states at specific external conditions, especially for high excitation intensities. This allowed us to establish analytical expressions of the lux-lux and lux-ampere characteristics and use them in analyzing the type of recombination mechanism, which corresponds to certain dependence of luminescence or photoconductivity on the excitation intensity.     

Topological and dynamical excitations in a classical 2D easy-axis Heisenberg model

The properties of dynamical solitons (magnon droplets) in the classical, two-dimensional anisotropic Heisenberg model with easy-axis exchange anisotropy are studied. The solution of the Landau-Lifshitz equation in the continuum limit for the soliton with topological charge q = 1 is obtained numerically using a shooting method. We analized a wide range of the anisotropy parameter and our results are in good agreement with results obtained from spin dynamics simulations. The dependence of an internal precession frequency of the soliton on both the anisotropy parameter and the radius of the soliton is also investigated. Finally, the limits of applicability of the continuum approach are discussed. Received 22 August 2000     

An integrable 3D model for Heisenberg ferromagnetic spin system with biquadratic interactions

We formulate a model Hamiltonian to a 3D Ferromagnetic spin system incorporating biquadratic interactions. The dynamics is represented by a higher order (3+1) dimensional integrable nonlinear Schrödinger equation. We construct the Lax pair associate with the system and find multisoliton solutions using Darboux transformation(DT). We bilinearize the equation using Hirota’s bilinearization procedure and find one soliton solution.     

Dispersive crystal field excitations and quadrupolar interactions in UPd3

We report inelastic neutron scattering measurements and random phase approximation calculations of the dispersive crystal field excitations of UPd(3). The measured spectra at lower energies agree with those calculated using quadrupolar interaction parameters deduced from bulk and x-ray scattering measurements. The more intense excitations arising from the hexagonal sites were used to obtain exchange parameters which proved to be anisotropic.     

Ground-state energy of the exciton-phonon system in a magnetic field

Upper bounds for the ground-state energy of the exciton-phonon system in a magnetic field are calculated variationally using a wavefunction whose symmetry is suited both to low and high magnetic fields. The displacement amplitudes for the exciton-phonon interaction are those known from the free field case. The results compare satisfactorily with experimental data for TlCl and TlBr.     

Localized magnetic excitations of coupled impurities in a transverse Ising ferromagnet

A Green's function formalism is used to calculate the spectrum of excitations of two neighboring impurities implanted in a semi-infinite ferromagnetic. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field and results are given for the effect of the exchange coupling, position and orientation of the impurities on the spectra of localized spin wave modes.     

Collective excitations in a system of excitons

A high density system of Frenkel excitons is investigated under the condition that no type of condensation occurs in the system. The dynamic interaction between excitons and the excitonphonon interaction are considered. It is shown that sound-like collective excitations can exist in systems of excitons and excitons and phonons. The damping of excitations caused by finite life time of excitons is also taken into account.The author wishes to express his thanks to Prof. M.Trlifaj for suggesting the present theme and for useful discussions.     

Unitarity bounds in the vector condensate model of electroweak interactions

We replace the standard model scalar doublet by a doublet of vector fields and generate masses by dynamical symmetry breaking. Oblique radiative corrections are small if the new vector bosons (B ⁺,B ⁰) are heavy. In this note it is shown that the model has a low momentum scale and above Λ?2 TeV it does not respect the perturbative unitarity. From tree-graph unitarity the allowed region ofB ⁺ (B ⁰) mass is estimated asm _B ⁺≥333 GeV (m _B ⁰≥373 GeV) at Λ=1 TeV.     

Bands of Localized Electromagnetic Waves in 3D Random Media

Anderson localization of electromagnetic waves in three-dimensional disordered dielectric structures is studied using a simple yet realistic theoretical model. An effective approach based on analysis of probability distributions, not averages, is developed. The disordered dielectric medium is modeled by a system of randomly distributed electric dipoles. Spectra of certain random matrices are investigated and the possibility of appearance of the continuous band of localized waves emerging in the limit of an infinite medium is indicated. It is shown that localization could be achieved without tuning the frequency of monochromatic electromagnetic waves to match the internal (Mie-type) resonances of individual scatterers. A possible explanation for the lack of experimental evidence for strong localization in 3D as well as suggestions how to make localization experimentally feasible are also given. Rather peculiar requirements for setting in localization in 3D as compared to 2D are indicated.     

Localized magnetic excitations of coupled impurities in a Heisenberg ferromagnet: Optical defect modes

A Green's function method is used to obtain the spectrum of excitations of two neighboring impurities in a semi-infinite ferromagnet. The equations of motion for Green's functions are determined in the framework of the Heisenberg model. The energies of non-resonant localized modes are calculated as a function of the interaction parameters for the exchange coupling between impurity-spin pairs, host-spin pairs, and impurity-host neighbors, as well as the effective field parameter at the impurity sites. With two impurities the system is less symmetric and has more localized modes when compared with a single impurity case.     

Localized magnetic excitations for a line of magnetic impurities in a transverse Ising thin film ferromagnet

A Green's function method is used to obtain the spectrum of spin excitations associated with a linear array of magnetic impurities implanted in a ferromagnetic thin film. The equations of motion for the Green's functions of the anisotropic film are written in the framework of the Ising model in a transverse field. The frequencies of localized modes are calculated as a function of the interaction parameters for the exchange coupling between impurity-spin pairs, host-spin pairs, and impurity-host neighbors, as well as the effective field parameter at the impurity sites.     

Substrate interactions, effects of symmetry breaking, and convection in a 2D horizontally shaken granular system

We describe experiments on a horizontally shaken [x = Asin(omegat)] single layer of hard spheres rolling on a nearly horizontal surface. We identify a novel substrate-mediated convective flow which occurs when the system is tilted slightly so that the weak gravitational force, g-->(eff), acting on the particles is not parallel to the driving direction. As the shaking amplitude is increased, the system progresses through four regimes: solid-flat, solid-inclined, convective, and disordered. The control parameter is the driving velocity, Aomega, rather than the usual Aomega(2) of vertically shaken 3D systems. At the onset of convection, the critical velocity is V(c) approximately sqrt[2g(eff)d].     

Dual model for magnetic excitations and superconductivity in UPd 2 Al 3

The Heavy Fermion state in UPd₂Al₃ may be approximately described by a dual model where two of the three U-5 f electrons are in a localized state split by the crystalline electric field into two low lying singlets with a splitting energy Δ≃ 6 meV. The third 5 f electron has itinerant character and forms the Heavy Electron bands. Inelastic neutron scattering and tunneling experiments suggest that magnetic excitons, the collective propagating crystal field excitations of the localized 5 f electrons, mediate superconducting (sc) pairing in UPd₂Al₃. A theory for this novel mechanism is developed within a nonretarded approach. A model for the magnetic exciton bands is analyzed and compared with experiment. The sc pair potential which they mediate is derived and the gap equations are solved. It is shown that this mechanism favors an odd parity state which is nondegenerate due to the combined symmetry breaking by the crystalline electric field and the AF order parameter. A hybrid model including the spin fluctuation contribution to the pairing is also discussed. Received 22 October 2001 and Received in final form 28 February 2002     

K－p scattering and hyperon excitations in a chiral quark model

A chiral quark-model approach is employed to study theˉ KN scattering at low energies.The processes of Kˉp → Σ 0 π 0,Λπ 0 andˉ K 0 n at P K 800 MeV/c （i.e.the center mass energy W 1.7 GeV） are investigated.The analysis shows that the Λ（1405）S 01 dominates the processes Kˉp → Σ 0 π 0,ˉ K 0 n in the energy region considered here.Around P K 400 MeV/c,the Λ（1520）D 03 is responsible for a strong resonant peak in the cross section of Kˉp → Σ 0 π 0 andˉ K 0 n.To reproduce the data,an unexpectedly large coupling for Λ（1520）D 03 to KN is needed.In contrast,the coupling for Λ（1670）S 01 to KN appears to be weak,which could be due to configuration mixings between Λ（1405）S 01 and Λ（1670）S 01 .By analyzing Kˉ p → Λπ 0,evidences for two low mass S-wave states,Σ（1480）S 11 and Σ（1560）S 11,seem to be available.With these two states,the reaction Kˉp →ˉ K 0 n can also be described well.However,it is difficult to understand the low masses of Σ（1480）S 11 and Σ（1560）S 11 .The s-channel amplitudes for Kˉp → Λπ 0 are also larger than the naive quark model expectations.The non-resonant background contributions,i.e.t-channel and/or u-channel,also play important roles in the explanation of the angular distributions due to amplitude interferences.