Static and dynamical distortions of helium clusters

We examine the effects of impurities (doping) and rotational excitation on the structural and energetic properties of helium clusters. Quantum Monte Carlo (QMC) techniques are used to study ground and rotationally excited states of pure and doped clusters. We use exponentially correlated wave functions and treat the molecular impurities as rigid. Whereas pure He_N show essentially monotonic decay of density from a central maximum value, addition of impurities induces local ordering of He to an extent dependent on the impurity-He binding. Rotational excitation of He_N gives rise to extremely large centrifugal distortions. The location of impurities also appears to change upon rotational excitation. The implications of these distortions on impurity spectra are discussed for SF₆He_N, and compared to recent experimental results.     

Electron states in metal clusters

Radiative transitions in metal clusters are analyzed in terms of quantum transitions of valence electrons that interact with surrounding valence electrons and ion cores. The analysis is based on the solution of the Thomas-Fermi equation for valence electrons in a spherical cluster. The quantum states of valence electrons and the energy and the dipole moments of transitions are determined in the quasiclassical approximation. It is shown that the frequencies of dipole oscillations and the dipole moments of the transitions strongly depend on the size of a cluster.     

Ordered systems of ultrafine ferromagnetic particles

A simple model for systems of dipolarly interacting single-domain ultrafine ferromagnetic particles is studied by Monte Carlo simulations. Zero field cooling and field cooling as well as relaxation experiments are used to compare systems with positional and orientational disorder to systems which are (i) positionally, (ii) orientationally, and (iii) positionally and orientationally ordered. It is shown that, as far as macroscopic observables are concerned, these partially [cases (i) and (ii)] or fully [case (iii)] ordered systems, despite quantitative differences, behave qualitatively very similar to the disordered one. This holds true even for the relaxation, where the decay of the magnetization M(t)/M_S (measured in units of the saturation magnetization M_S) leads to an instantaneous relaxation rate W(t) = -d/dt ln [ M(t)/M_S ] vanishing as a power-law as a function of time t, W(t) ∝t^-n. The exponent n is found to increase with increasing concentration, and becomes n > 1 for dense systems.     

Ordered and disordered dynamics in monolayers of rolling particles

We consider the ordered and disordered dynamics for monolayers of rolling self-interacting particles modeling water molecules. The rolling constraint represents a simplified model of a strong, but rapidly decaying bond with the surface. We show the existence and nonlinear stability of ordered lattice states, as well as disturbance propagation through and chaotic vibrations of these states. We study the dynamics of disordered gas states and show that there is a surprising and universal linear connection between distributions of angular and linear velocity, allowing definition of temperature.     

Quasi-energy of single quantum particles and a Bose-Einstein condensate in a dynamical trap

The quasi-energy states have been found analytically for single quantum particles and an atomic Bose-Einstein condensate in a trap with periodically oscillating walls with a small modulation depth. A resonance is shown to exist as the modulation frequency approaches the difference of the frequencies corresponding to the levels in the unperturbed problem. Quasi-energy splitting and, accordingly, beats with a periodic population exchange between two levels in resonance have been found in the resonant case. Bistability of the response to trap size modulation, when the sustenance (depending on the initial conditions) of various quasi-energy states is possible under the same conditions, has been found for a Bose-Einstein condensate under resonance conditions.     

Ordered and disordered states in magnetically diluted insulating systems

The different kinds of magnetic phases arising in insulating systems are described and discussed. Attention is focussed on semi-disordered phases which retain partially long range order as reentrant properties and local canted states, and disordered phases without long range order as spin glasses and spin cluster states. The properties of some systems are described, choosen among those where Mössbauer spectroscopy has been performed, mainly spinels, glasses, fluorides, Eu-based compounds. Some unsolved problems are shortly pointed out.     

Gibbs states and equilibrium states for finitely presented dynamical systems

We studyfinitely presented dynamical systems (which generalize Axiom A systems) and show that the notions of equilibrium states and Gibbs states (for Hölder continuous functions) are equivalent. Our results extend those of Ruelle, Haydn, and others on Axiom A dynamical systems and statistical mechanics.     

Ordered valence-bond states in symmetric two-dimensional spin-orbital systems

We consider a superexchange Hamiltonian, H = -SUM ()(2S(i) . S(j)-(1/2)) (2T(i) . T(j)-(1/2)), which describes systems with orbital degeneracy and strong electron-phonon coupling in the limit of large on-site repulsion. In an SU(4) Schwinger boson representation, a reduced spin-orbital interaction is derived exactly, and a mean field theory has been developed. In one dimension, a spin-orbital liquid state with a finite gap is obtained. On a two-dimensional square lattice a novel type of spin-orbital ferromagnetically ordered state appears, while spin and orbital are antiferromagnetic. An important relation has been found, relating the spin and orbital correlation functions to the combined spin-orbital ones.     

Exotic states in the dynamical Casimir effect

Inhibited decoherence has been recently observed in a dissipative two-level system by increasing the strength of the coupling with the reservoir. The system is described by the spin-boson model under a perturbation approach in the delocalized phase regime occurring in weak-coupling limit at zero temperature. Within this scenario, persistence of coherence is found over long times for various low frequency structures of the bosonic environment near a band gap. Special resonances provoke transitions in the long time dynamics if the transition amplitude of the two-level system is greater than the band gap frequency or in absence of any band gap. These transitions may hinder the the loss of coherence in the spin-boson model. Limitations of the approximations are also discussed.     

Ordered states and structural transitions in a system of Abrikosov vortices with periodic pinning

A system of Abrikosov vortices in a quasi-two-dimensional HTSC plate is considered for various periodic lattices of pinning centers. The magnetization and equilibrium configurations of the vortex density for various values of external magnetic field and temperature are calculated using the Monte Carlo method. It is found that the interaction of the vortex system with the periodic lattice of pinning centers leads to the formation of various ordered vortex states through which the vortex system passes upon an increase or a decrease in the magnetic field. It is shown that ordered vortex states, as well as magnetic field screening processes, are responsible for the emergence of clearly manifested peaks on the magnetization curves. Extended pinning centers and the effect of multiple trapping of vortices on the behavior of magnetization are considered. Melting and crystallization of the vortex system under the periodic pinning conditions are investigated. It is found that the vortex system can crystallize upon heating in the case of periodic pinning.     

Noncollinear states in a chain of single-domain magnetic particles

The ground state of a chain of single-domain magnetic particles has been theoretically analyzed. The conditions under which this state corresponds to a noncollinear structure in zero external magnetic field are determined. Such noncollinear states are due to the features of the long-range magnetostatic interactions in the systems without the center of inversion.     

Correlated motion of two particles in a fluid

Conditional velocity cross correlation functions of the form <v_i (0)v_j (t); r_ij (0)> in the Lennard-Jones fluid are investigated by molecular dynamics simulation. As shown in previous work, these cross correlation functions may be related to memory functions in a similar manner as the usual velocity auto-correlation function. To compute the memory functions, a modified version of Detyna and Singer's algorithm has been used.     

Generalized definition of coherent states and dynamical groups

It is shown that coherent states may be defined for an arbitrary dynamical (Hamiltonian) quantum system and the definition is consistent with the requirement that the Hamiltonian commutes with a Lie algebra γ, and γ can be integrated to form a Lie groupG.     

Convection in a vibrated granular layer

When a granular layer is submitted to an oscillating acceleration with a peak value larger than gravity, a large scale motion develops. This movement is in some ways similar to the one displayed by a liquid heated from below, and it is called granular convection. Different conditions beside the parameters of the forcing can affect it, such as the presence of an interstitial gas or the roughness of the walls. We have carried out an experiment to study the convective movement of a granular layer with a temporal resolution high enough to describe the motion of individual grains within one oscillating period. We also present experimental results concerning the friction that the lateral walls exert on the grains and its relevance on granular convection.     

Localized states in a plane film containing impurity particles

Processes of the formation of localized excitations in a two-dimensional matrix consisting of aggregates and impurity particles are studied theoretically. The spatial dispersion determined by the dipole-dipole interaction of aggregates in a quasi-discrete medium with cubic nonlinearity is taken into account. It is shown within the framework of simple models that the longest lived excitations of the matrix are localized near particles.