Gap soliton dynamics in an optical lattice as a parametrically driven pendulum

A long wavelength optical lattice is generated in a two-level medium by low-frequency contrapropagating beams. Then a short wave length gap soliton generated by evanescent boundary instability (supratransmission) undergoes a dynamics shown to obey the Newton equation of the parametrically driven pendulum, hence presenting extremely rich, possibly chaotic, dynamical behavior. The theory is sustained by numerical simulations and provides an efficient tool to study soliton trajectories

Three-dimensional harmonic oscillator and time evolution in quantum mechanics

The problem of defining time (or phase) operator for three-dimensional harmonic oscillator has been analyzed. A new formula for this operator has been derived. The results have been used to demonstrate a possibility of representing quantum-mechanical time evolution in the framework of an extended Hilbert space structure. Physical interpretation of the extended structure has been discussed shortly, too.Comment: 14 pages; submitted to Phys Rev

Development of superconducting YBa2Cu3O(x) wires with low resistance electrical contacts

Materials exhibiting superconductivity above liquid nitrogen temperatures (77 K) will enable new applications of this phenomena. One of the first commercial applications of this technology will be superconducting magnets for medical imaging. However, a large number of aerospace applications of the high temperature superconducting materials have also been identified. These include magnetic suspension and balance of models in wind tunnels and resistanceless leads to anemometers. The development of superconducting wires fabricated from the ceramic materials is critical for these applications. The progress in application of a patented fiber process developed by Clemson University for the fabrication of superconducting wires is reviewed. The effect of particle size and heat treatment on the quality of materials is discussed. Recent advances made at Christopher Newport College in the development of micro-ohm resistance electrical contacts which are capable of carrying the highest reported direct current to this material is presented

Coherent spin relaxation in molecular magnets

Numerical modelling of coherent spin relaxation in nanomagnets, formed by magnetic molecules of high spins, is accomplished. Such a coherent spin dynamics can be realized in the presence of a resonant electric circuit coupled to the magnet. Computer simulations for a system of a large number of interacting spins is an efficient tool for studying the microscopic properties of such systems. Coherent spin relaxation is an ultrafast process, with the relaxation time that can be an order shorter than the transverse spin dephasing time. The influence of different system parameters on the relaxation process is analysed. The role of the sample geometry on the spin relaxation is investigated.Comment: Latex file, 22 pages, 7 figure

Nonlinear spin relaxation in strongly nonequilibrium magnets

A general theory is developed for describing the nonlinear relaxation of spin systems from a strongly nonequilibrium initial state, when, in addition, the sample is coupled to a resonator. Such processes are characterized by nonlinear stochastic differential equations. This makes these strongly nonequilibrium processes principally different from the spin relaxation close to an equilibrium state, which is represented by linear differential equations. The consideration is based on a realistic microscopic Hamiltonian including the Zeeman terms, dipole interactions, exchange interactions, and a single-site anisotropy. The influence of cross correlations between several spin species is investigated. The critically important function of coupling between the spin system and a resonant electric circuit is emphasized. The role of all main relaxation rates is analyzed. The phenomenon of self-organization of transition coherence in spin motion, from the quantum chaotic stage of incoherent fluctuations, is thoroughly described. Local spin fluctuations are found to be the triggering cause for starting the spin relaxation from an incoherent nonequilibrium state. The basic regimes of collective coherent spin relaxation are studied.Comment: Latex file, 31 page

Local-field correction to one- and two-atom van der Waals interactions

Based on macroscopic quantum electrodynamics in linearly and causally responding media, we study the local-field corrected van der Waals potentials and forces for unpolarized ground-state atoms placed within a magnetoelectric medium of arbitrary size and shape. We start from general expressions for the van der Waals potentials in terms of the (classical) Green tensor of the electromagnetic field and the atomic polarizability and incorporate the local-field correction by means of the real-cavity model. In this context, special emphasis is given to the decomposition of the Green tensor into a medium part multiplied by a global local-field correction factor and, in the single-atom case, a part that only depends on the cavity characteristics. The result is used to derive general formulas for the local-field corrected van der Waals potentials and forces. As an application, we calculate the van der Waals potential between two ground-state atoms placed within magnetoelectric bulk material.Comment: 9 pages, 2 figures, corrections according to erratu

Способ определения изометричности расположения бедренного и большеберцового тоннелей до их формирования при пластике передней крестообразной связки

Relevance. One of the key points in plasty of the anterior cruciate ligament (ACL) is the isometric position of the graft, in which its tension remains the same during flexion and extension of the knee joint. However, no method has been described today for the intraoperative determination of the isometricity of the location of the femoral and tibial tunnels (for placing the graft) before their formation.Purpose of the study. To develop a method for intraoperative determination of the isometricity of the location of the femoral and tibial tunnels before their formation during ACL plasty and to study its effectiveness.Material and methods. The study included 30 patients who underwent ACL repair. For a preliminary intraoperative assessment of the isometric areas of graft fixation on the femur and tibia, the proposed original method with the use of two knot pushers and a thread passing through them. Isometry was assessed by the degree of displacement of this thread. After determining the isometric areas of fixation, the femoral and tibial tunnels were formed, and the isometric position of the graft before its fixation in the tibia was checked by the degree of displacement of the threads with which the graft was sutured relative to the aperture of the tibial tunnel.Results. The average displacement of the thread relative to the pusher of the knot in the preliminary determination of the isometric areas of fixation according to the proposed method corresponded to the displacement of the threads with which the distal end of the graft was sewn relative to the outer aperture of the tibial tunnel (this value did not exceed 2 mm on average) until the final fixation of the graft in the tibial tunnel.Conclusion. The developed method makes it possible to determine the isometric location of the femoral and tibial tunnels during arthroscopic plasty of the anterior cruciate ligament before their formation. If the location of the fixation points on the femur and tibia is determined non-isometric, it is possible to correct their position.Актуальность. Одним из ключевых моментов при пластике передней крестообразной связки (ПКС) является изометричное расположение трансплантата, при котором его натяжение остается одинаковым при сгибании и разгибании в коленном суставе. Тем не менее на сегодняшний день не описано способа интраоперационного определения изометричности расположения бедренного и большеберцового тоннелей (для установки трансплантата) до их формирования.Цель исследования. Разработать способ интраоперационного определения изометричности расположения бедренного и большеберцового тоннелей до их формирования при пластике ПКС и изучить его эффективность.Материал и методы. В исследование были включены 30 пациентов, которым выполнялась пластика ПКС. Для предварительной интраоперационной оценки изометричных областей фиксации трансплантата на бедренной и большеберцовой костях использовали предложенный оригинальный способ с применением двух толкателей узла и проходящей через них нити. Изометрию оценивали по степени смещения этой нити. После определения изометричных областей фиксации формировали бедренный и большеберцовый тоннели и проверяли изометричность положения трансплантата до его фиксации в большеберцовой кости по степени смещения нитей, которыми прошит трансплантат, относительно апертуры большеберцового тоннеля.Результаты. Средняя величина смещения нити относительно толкателя узла при предварительном определении изометрических областей фиксации по предложенному способу соответствовала величине смещения нитей, которыми прошит дистальный конец трансплантата, относительно наружной апертуры большеберцового тоннеля (данная величина в среднем не превышала 2 мм) до окончательной фиксации трансплантата в большеберцовом тоннеле.Вывод. Разработанный метод позволяет определить изометричное расположение бедренного и большеберцового тоннелей при артроскопической пластике передней крестообразной связки до их формирования. В случае определения неизометричности расположения точек фиксации на бедренной и большеберцовой костях возможна корректировка их положения

Scattering of slow-light gap solitons with charges in a two-level medium

The Maxwell-Bloch system describes a quantum two-level medium interacting with a classical electromagnetic field by mediation of the the population density. This population density variation is a purely quantum effect which is actually at the very origin of nonlinearity. The resulting nonlinear coupling possesses particularly interesting consequences at the resonance (when the frequency of the excitation is close to the transition frequency of the two-level medium) as e.g. slow-light gap solitons that result from the nonlinear instability of the evanescent wave at the boundary. As nonlinearity couples the different polarizations of the electromagnetic field, the slow-light gap soliton is shown to experience effective scattering whith charges in the medium, allowing it for instance to be trapped or reflected. This scattering process is understood qualitatively as being governed by a nonlinear Schroedinger model in an external potential related to the charges (the electrostatic permanent background component of the field).Comment: RevTex, 14 pages with 5 figures, to appear in J. Phys. A: Math. Theo

Origin of Pure Spin Superradiance

The question addressed in this paper is: What originates pure spin superradiance in a polarized spin system placed inside a resonator? The term "pure" means that no initial coherence is imposed on spins, and its appearance manifests a purely self-organized collective effect. The consideration is based on a microscopic model with dipole spin interactions. An accurate solution of evolution equations is given. The results show that the resonator Nyquist noise does not play, contrary to the common belief, any role in starting spin superradiance, but the emergence of the latter is initiated by local spin fluctuations. The decisive role of nonsecular dipole interactions is stressed.Comment: 1 file, 13 pages, RevTe

Body-assisted van der Waals interaction between two atoms

Using fourth-order perturbation theory, a general formula for the van der Waals potential of two neutral, unpolarized, ground-state atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is derived. The theory is applied to two atoms in bulk material and in front of a planar multilayer system, with special emphasis on the cases of a perfectly reflecting plate and a semi-infinite half space. It is demonstrated that the enhancement and reduction of the two-atom interaction due to the presence of a perfectly reflecting plate can be understood, at least in the nonretarded limit, by using the method of image charges. For the semi-infinite half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure