arXiv subjects

V. I. Yukalov

Publications and source records attributed to V. I. Yukalov.

At least 307 records · Page 17Linked to original sources

arXiv1999

Nonlinear Dynamics of Nuclear-Electronic Spin Processes in Ferromagnets

Spin dynamics is considered in ferromagnets consisting of electron and nuclear subsystems interacting with each other through hyperfine forces. In addition, the ferromagnetic sample is coupled with a resonance electric circuit. Under these conditions, spin relaxation from a strongly nonequilibrium initial state displays several peculiarities absent for the standard set-up in studying spin relaxation. The main feature of the nonlinear spin dynamics considered in this communication is the appearance of ultrafast coherent relaxation, with characteristic relaxation times several orders shorter than the transverse relaxation time $T_2$. This type of coherent spin relaxation can be used for extracting additional information on the intrinsic properties of ferromagnetic materials and also can be employed for different technical applications.

cond-mat↗

arXiv1999

Weighted Fixed Points in Self-Similar Analysis of Time Series

The self-similar analysis of time series is generalized by introducing the notion of scenario probabilities. This makes it possible to give a complete statistical description for the forecast spectrum by defining the average forecast as a weighted fixed point and by calculating the corresponding a priori standard deviation and variance coefficient. Several examples of stock-market time series illustrate the method.

cond-mat↗

arXiv1999

Nonlinear Spin Dynamics in Ferromagnets with Electron-Nuclear Coupling

Nonlinear spin motion in ferromagnets is considered with nonlinearity due to three factors: (i) the sample is prepared in a strongly nonequilibrium state, so that evolution equations cannot be linearized as would be admissible for spin motion not too far from equilibrium, (ii) the system considered consists of interacting electron and nuclear spins coupled with each other via hyperfine forces, and (iii) the sample is inserted into a coil of a resonant electric circuit producing a resonator feedback field. Due to these nonlinearities, coherent motion of spins can develop, resulting in their ultrafast relaxation. A complete analysis of mechanisms triggering such a coherent motion is presented. This type of ultrafast coherent relaxation can be used for studying intrinsic properties of magnetic materials.

cond-mat↗

arXiv1999

Collimation Mechanism for Atom Lasers

A mechanism is suggested for creating well-collimated beams of neutral spin-polarized particles by means of magnetic fields. This mechanism can be used in atom lasers for the formation of directed coherent beams of atoms. The directed motion of atoms is achieved only with the help of magnetic fields, no mechanical collimators being involved.

cond-mat↗

arXiv1999

Cannon for Neutral Particles

Dynamics of spin-polarized neutral particles, such as neutrons or neutral atoms and molecules, in magnetic fields is studied. A new regime of motion is found where particles move mainly in one direction forming a well-collimated beam. This regime suggests a mechanism for creating devices emitting directed beams of neutral particles.

cond-mat↗

arXiv1999

Anomalous Transient Current in Nonuniform Semiconductors

Nonequilibrium processes in semiconductors are considered with highly nonuniform initial densities of charge carriers. It is shown that there exist such distributions of charge densities under which the electric current through a sample displays quite abnormal behaviour flowing against the applied voltage. The appearance of this negative electric current is a transient phenomenon occurring at the initial stage of the process. After this anomalous negative fluctuation, the electric current becomes normal, i.e. positive as soon as the charge density becomes more uniform. Several possibilities for the practical usage of this effect are suggested.

cond-mat↗

arXiv1999

Coherent Polariton Radiation and Light Localization

A system of resonance atoms is placed in a medium with developed polariton effect. In the spectrum of polariton states there can exist a band gap. If an atom with a resonance frequency inside the polariton gap is incorporated into the medium, the atomic spontaneous emission can be strongly suppressed. This is what is called the localization of light. Nevertheless, an ensemble of resonance atoms inside the polariton gap can radiate if their coherent interaction is sufficiently strong. Conditions when coherent polariton radiation can appear and the properties of this coherent polariton emission are studied.

cond-mat↗

arXiv1999

Conditions for Nuclear-Matter Lasers

Conditions are analysed when in dense and hot nuclear matter large amounts of Bose particles can be created. An intensive production of Bose particles is the main necessary condition for realizing their coherent emission similar to radiation from photon lasers. The consideration is based on the multichannel model of nuclear matter. Analysis shows that possible candidates for nuclear--matter lasing are mesons (mainly pions), dibaryons, and gluons.

hep-ph↗

arXiv1999

Interplay between Mesoscopic and Microscopic Fluctuations in Ferromagnets

A model of a ferromagnet is considered, in which there arise mesoscopic fluctuations of paramagnetic phase. The presence of these fluctuations diminishes the magnetization of the ferromagnet, softens the spin-wave spectrum, increases the spin-wave attenuation, shortens the magnon free path, lowers the critical point, and can change the order of phase transition. A special attention is paid to the interplay between these mesoscopic paramagnetic fluctuations and microscopic fluctuations due to magnons. One of the main results of this interplay is an essential extension of the region of parameters where the ferromagnet-paramagnet phase transition is of first order.

cond-mat.stat-mech↗

arXiv1999

Stabilizing Role of Mesoscopic Fluctuations in Spin Systems

The occurrence of mesoscopic fluctuations in statistical systems implies, from the point of view of dynamical theory, the existence of local instabilities. However, the presence of such fluctuations can make a system, as a whole, more stable from the thermodynamic point of view. Thus, in many cases, a local dynamic instability is a requisite for the global thermodynamic stability. This idea is illustrated by several spin models.

cond-mat.stat-mech↗

arXiv1998

Statistical Green's Functions

The basic mathematical properties of Green's functions used in statistical mechanics as well as the equations defining these functions and the techniques of solving these equations are reviewed. An approach is presented called the Correlated Iteration Theory, which has been developed by the author. This approach differs from all other known variants of perturbation theory for Green's functions by the combination of two factors: the systematic formulation of an algorithm for obtaining subsequent approximations and the consistent consideration of interparticle correlations at each step of the procedure.

cond-mat.stat-mech↗

arXiv1998

Spin Maser under Stationary Pumping

Spin dynamics of a polarized spin system is studied when the latter is coupled with a resonant electric circuit and is under the action of an external pumping supporting a stationary nonequilibrium magnetization. A complete classification of possible regimes of spin motion is given. In addition to seven regimes considered earlier, two other transient regimes are found and thoroughly described: One is an oscillatory regime, when spins always move coherently but the degree of coherence fluctuates with time. Another is a pulsing regime, when spins reveal coherent motion during short pulses separated from each other by intervals of incoherent motion. These regimes are, in principle, transient, although may be extremely long lasting; their duration may be several orders longer than the transverse relaxation time and twice longer than the longitudinal relaxation time. Both transient regimes end with a coherent quasistationary regime.

cond-mat.stat-mech↗

arXiv1998

Booms and Crashes in Self-Similar Markets

Sharp changes in time series representing market dynamics are studied by means of the self--similar analysis suggested earlier by the authors. These sharp changes are market booms and crashes. Such crises phenomena in markets are analogous to critical phenomena in physics. A simple classification of the market crisis phenomena is given.

cond-mat.stat-mech↗

arXiv1998

Unified Approach to Crossover Phenomena

A general analytical method is developed for describing crossover phenomena of arbitrary nature. The method is based on the algebraic self-similar renormalization of asymptotic series, with control functions defined by crossover conditions. The method can be employed for such difficult problems for which only a few terms of asymptotic expansions are available, and no other techniques are applicable. As an illustration, analytical solutions for several important physical problems are presented.

cond-mat.stat-mech↗

arXiv1998

Self-Similar Exponential Approximants

An approach is suggested defining effective sums of divergent series in the form of self-similar exponential approximants. The procedure of constructing these approximants from divergent series with arbitrary noninteger powers is developed. The basis of this construction is the self-similar approximation theory. Control functions governing the convergence of exponentially renormalized series are defined from stability and fixed-point conditions and from additional asymptotic conditions when the latter are available. The stability of the calculational procedure is checked by analyzing cascade multipliers. A number of physical examples for different statistical systems illustrate the generality and high accuracy of the approach.

cond-mat.stat-mech↗

arXiv1998

Self-Similar Interpolation in Quantum Mechanics

An approach is developed for constructing simple analytical formulae accurately approximating solutions to eigenvalue problems of quantum mechanics. This approach is based on self-similar approximation theory. In order to derive interpolation formulae valid in the whole range of parameters of considered physical quantities, the self-similar renormalization procedure is complimented here by boundary conditions which define control functions guaranteeing correct asymptotic behaviour in the vicinity of boundary points. To emphasize the generality of the approach, it is illustrated by different problems that are typical for quantum mechanics, such as anharmonic oscillators, double-well potentials, and quasiresonance models with quasistationary states. In addition, the nonlinear Schrödinger equation is considered, for which both eigenvalues and wave functions are constructed.

cond-mat↗

arXiv1998

Renormalization--Group Solutions for Yukawa Potential

The self--similar renormalization group is used to obtain expressions for the spectrum of the Hamiltonian with the Yukawa potential. The critical screening parameter above which there are no bound states is also obtained by this method. The approach presented illustrates that one can achieve good accuracy without involving extensive numerical calculations, but invoking instead the renormalization--group techniques.

cond-mat↗

arXiv1998

Resummation Methods for Analyzing Time Series

An approach is suggested for analyzing time series by means of resummation techniques of theoretical physics. A particular form of such an analysis, based on the algebraic self-similar renormalization, is developed and illustrated by several examples from the stock market time series.

cond-mat.stat-mech↗