Intermode Interaction Research Articles

First Order Phase Transition in Ferromagnetic Semiconductors in External Electric and Magnetic Fields (by the example of La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-) (on the example of La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-)

It is shown that during a first-order phase transition in ferromagnetic semiconductors based on lanthanum manganites, along with the semiconductor phase of s-electrons, a phase arises with fluctuation short-range order associated with a change in the sign of the intermode interaction parameter in the system of d-electrons. The phase separation region is characterized by fluctuations of local magnetization depending on temperature and external magnetic field. In this case, the temperature of the first order phase transition, above which phase separation arises, is appreciably affected by double exchange. Due to self-heating, the phase separation is significantly affected by the electric field, which leads to N-shaped current-voltage characteristics. Using La1-xCaxMnO3 as an example, it is shown that the switching on of electric and magnetic fields during a first-order phase transition in ferromagnetic semiconductors is accompanied by the appearance of self-oscillations of the electric current and magnetization. Keywords: double exchange, electron fluctuations of spin and charge density, Anderson localization, self-oscillations.

Фазовый переход первого рода в ферромагнитных полупроводниках во внешних электрическом и магнитном полях (на примере La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-)

It is shown that during a first-order phase transition in ferromagnetic semiconductors based on lanthanum manganites, along with the semiconductor phase of s-electrons, a phase arises with fluctuation short-range order associated with a change in the sign of the intermode interaction parameter in the system of d-electrons. The phase separation region is characterized by fluctuations of local magnetization depending on temperature and external magnetic field. In this case, the temperature of the first order phase transition, above which phase separation arises, is appreciably affected by double exchange. Due to self-heating, the phase separation is significantly affected by the electric field, which leads to N-shaped current-voltage characteristics. Using La1–xCaxMnO3 as an example, it is shown that the switching on of electric and magnetic fields during a first-order phase transition in ferromagnetic semiconductors is accompanied by the appearance of self-oscillations of the electric current and magnetization.

Coupling coefficient of two rotated cylindrical dielectric resonators disposed in free space

The coupled-mode theory is an established and important tool in the electromagnetic theory which describes the exchange of energy between modes of a given system. In the framework of this theory, the coupling coefficient is the main parameter that quantifies the intermode interaction strength. In this paper, we introduce practical formulas for calculating the mutual coupling coefficient between two cylindrical dielectric resonators arbitrarily oriented in free space relative to each other. The correctness of the solution is validated against the full-wave numerical simulations performed with the COMSOL Multiphysics electromagnetic solver. In particular, the obtained formulas are suitable to analyze the gain/directivity and transmission/reflection properties of dielectric antennas and metasurfaces consisting of a large but finite number of dielectric resonators.

Exact solution of a non-stationary cavity with one intermode interaction

A non-stationary one-dimensional cavity can be described by the time-dependent and multi-mode effective Hamiltonian of the so-called dynamical Casimir effect. Due to the non-adiabatic boundary conditions imposed in one of the cavity mirrors, this effect predicts the generation of real photons out of vacuum fluctuations of the electromagnetic field. Such photon generation strongly depends on the number of modes in the cavity and their intermode couplings. Here, by using an algebraic approach, we show that for any set of functions parameterizing the effective Hamiltonian, the corresponding time-dependent Schrödinger equation admits an exact solution when the cavity has one intermode interaction. With the exact time evolution operator, written as a product of eleven exponentials, we obtain the average photon number in each mode, a few relevant observables, and some statistical properties for the evolved vacuum state.

Effect of Dispersion-Enhanced Sensitivity in a Two-Mode Optical Waveguide with an Asymmetric Diffraction Grating.

Analysis of trends in the development of silicon photonics shows the high efficiency regarding the creation of optical sensors. The concept of bimodal sensors, which suggests moving away from the usual paradigm based only on single-mode waveguides and using the inter-mode interaction of guided optical waves in a two-mode optical waveguide, is developed in the present paper. In this case, the interaction occurs in the presence of an asymmetric periodic perturbation of the refractive index above the waveguide surface. Such a system has unique dispersion properties that lead to the implementation of collinear Bragg diffraction with the mode number transformation, in which there is an extremely high dependence of the Bragg wavelength on the change in the refractive index of the environment. This is called the “effect of dispersion-enhanced sensitivity”. In this paper, it is shown by numerical calculation methods that the effect can be used to create optical sensors with the homogeneous sensitivity higher than 3000 nm/RIU, which is many times better than that of sensors in single-mode waveguide structures.

Индуцированные флуктуациями фазовые переходы и скирмионы в сильно коррелированных Fe-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-Si с нарушенной кристаллической структурой типа В20

Concentration and temperature transitions in strongly correlated Fe1-хCoxSi alloys with the Dzyaloshinsky-Moriyya (DM) interaction are considered within the framework of the spin-fluctuation theory and taking into account the results of LDA+U+SO calculations of the density of electronic states. It is shown that order-order concentration transitions with a change in the sign of the left-handed spin chirality (at x <0.2) to right-handed (at 0.2≤x<0.65), and again to left-handed, are associated with a change in the sign of the intermode interaction parameter. At the same time, it was indicated that in the region of compositions Fe1-хCoxSi 0.2≤x<0.65, temperature-prolonged first-order phase transitions appear, accompanied by the appearance of an intermediate temperature region of spin short-range order with uncompensated local magnetization and DM interaction. In an external magnetic field in this temperature range at 0.2<x<0.65, skyrmion microstructures appear. At x=0.65, the intermode interaction parameter becomes zero, and the long-range order becomes ferromagnetic (since the experiment implies compensation for the DM interaction). Instead of a prolonged first-order transition, a second-order temperature transition occurs at х=0.65. The constructed (h-T) diagrams of the magnetic states of Fe1-хCoxSi agree with the experiment.

Спиновые флуктуации и концентрационные магнитные переходы в киральных геликоидальных ферромагнетиках Fe-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-Si

The fluctuation theory is applied to the study of concentration transformations in chiral helicoidal ferromagnetic quasi-binary unordered Fe1-xCoxSi alloys with Dzyaloshinsky-Moriya interaction. The ground state is described on the basis of the LDA + U + SO approximations used in ab initio calculations with additional allowance for concentration fluctuations associated with the difference in the potentials of the intra-atomic Hubbard interaction at the sites occupied by iron and cobalt. Solutions of the obtained equations of the magnetic state for the phases of the long-range and short-range orders with the right and left magnetic chirality are considered. The concentration dependences of the parameters of the intermode interaction are investigated, and the regions of the compositions in which magnetic phase transitions of the first kind induced by thermal fluctuations take place, accompanied by the appearance of fluctuations of the spin helix. It is shown that the transition with a change of the sign of magnetic chirality is accompanied by the appearance of a minimum in the concentration dependence of the mode-mode parameter and the appearance of quantum helicoidal ferromagnetism with a noticeable increase in zero spin fluctuations.

Asymptotic Expansion of the Polarization Spectrum of a Two-Level System in a Polychromatic Field in the Small-Amplitude Limit

The impact of a polychromatic field on a two-level medium is considered in the limit of small amplitudes. In the indicated limit, we obtained asymptotic expansions of the polarization spectra of an arbitrary order of smallness. These polarization spectra were compared with the results of the numerical solution of the density matrix equation. Compared to numerical calculation methods, the asymptotic expansion offers an analytical estimation of the contribution of each individual component of a polychromatic field and a description of the intermode interaction. However, with an increase in the intensity of the driving field, the computational efficiency of the method decreases, since, to achieve the required accuracy, it is necessary to increase the order of the asymptotic expansion.

Асимптотическое разложение спектра поляризации двухуровневой системы в полихроматическом поле в пределе малых амплитуд

A two-level media driven by a polychromatic field is considered in the limit of small amplitudes. In this limit the asymptotic expansions for the polarization spectra of an arbitrary order are obtained. The obtained polarization spectra were compared with the numerical solution of the density matrix equation. Compared to numerical methods, the asymptotic expansion allows analytically estimating the contribution of each individual component of the polychromatic field and describing the intermode interaction. However, as the intensity of the acting field grows, the computational efficiency of the method decreases and to achieve the required accuracy it is necessary to increase the order of the asymptotic expansion.

Mode competition and hopping in optomechanical nano-oscillators

We investigate the inter-mode nonlinear interaction in the multi-mode optomechanical nano-oscillator which consists of coupled silicon nanocantilevers, where the integrated photonic crystal nanocavities provide the coupling between the optical and mechanical modes. Due to the self-saturation and cross-saturation of the mechanical gain, the inter-mode competition is observed, which leads to the bistable operation of the optomechanical nano-oscillator: only one of the mechanical modes can oscillate at any one time, and the oscillation of one mode extremely suppresses that of the other with a side mode suppression ratio (SMSR) up to 40 dB. In the meantime, mode hopping, i.e., the optomechanical oscillation switches from one mode to the other, is also observed and found to be able to be provoked by excitation laser fluctuations.

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.

Algebraic approach to electro-optic modulation of light: exactly solvable multimode quantum model

We theoretically study electro-optic light modulation based on the quantum model where the linear electro-optic effect and the externally applied microwave field result in the interaction between optical cavity modes. The model assumes that the number of interacting modes is finite and effects of the mode overlapping coefficient on the strength of the intermode interaction can be taken into account through dependence of the coupling coefficient on the mode characteristics. We show that, under certain conditions, the model is exactly solvable and, in the semiclassical approximation where the microwave field is treated as a classical mode, can be analyzed using the technique of the Jordan mappings for the su(2) Lie algebra. Analytical results are applied to study effects of light modulation on the frequency dependence of the photon counting rate. We also establish the conditions of validity of the semiclassical approximation by applying the methods of polynomially deformed Lie algebras for analysis of the model with quantized microwave field.

Impact of non-Gaussian statistics of atmospheric variables on extreme intramonth anomalies

The analysis of asymmetry of probability distribution functions (PDF) is carried out for key atmospheric variables using the JRA-55 reanalysis data in the troposphere of the Northern Hemisphere for 1976–2014. The nonzero asymmetry of the PDF indicates the deviation of the PDF from the normal distribution. The analysis was carried out for two time-scale intervals: synoptic variability (SV) of 2–7 days and low-frequency variability (LV) of 9–30 days. Statistically significant deviations from the normal probability distribution occur in the regions of the most frequent formation of atmospheric baroclinic perturbations, i.e., over the western parts of the oceans in midlatitudes and downstream in the atmosphere. In the SV time-scale interval, a negative asymmetry of the vertical velocity is revealed in isobaric coordinates for the entire thickness of the free troposphere, which agrees with the overall dominance of cyclonic anomalies in this interval of time scales. In the LV interval, the asymmetry of this variable in the entire free troposphere is positive, which indicates the dominance of anticyclonic anomalies at these time scales. For the zonal velocity, temperature, and geopotential, the asymmetry sign of the PDF for variability with time scales of 2–7 days is different for the upper and lower free troposphere. The asymmetry of the PDF for atmospheric variables indicates the important role of the intermode interaction in the formation of baroclinic perturbations. The corresponding deviations of synoptic variability from the normal distribution, which is found in the upper troposphere of the subpolar and polar latitudes, can be related to the interaction of these perturbations with the winter polar vortex. These deviations of PDF from the normal distribution substantially increase the probability of the appearance of large (in absolute value) anomalies as compared to the case of the Gaussian PDF.

Relaxation Process of Interacting Two-mode System Influenced by Markovian Thermal Reservoirs

Two different models of a relaxation process are considered for a linearly interacting two-mode system under the influence of independent Markovian thermal reservoirs. One is to describe the relaxation process of bare particles and the other is to describe the one of quasi particles which are derived from bare particles by the Bogoliubov transformation. The difference is that the former does not includes the effect of the inter-mode interaction on the damping operator while the latter does. The equations of motion are solved algebraically by making use of non-equilibrium thermo field dynamics. The relaxation processes in the two models are investigated in detail. The results are applied for investigating a non-ideal beam splitter with photon loss and noise addition.

Current-induced distortion of the band structure and formation of pseudogaps in magnonic crystals

Using numerical simulations, we have studied how electric current, passing along the periodicity direction in a lateral magnetic superlattice with modulated saturation magnetization, affects the propagation of magnetostatic surface spin waves (MSSWs) across it. It is shown that when the current flows against the normal lattice modes excited by a built-in antenna, it mediates excitation of new MSSW modes. These current-assisted modes are found to be co-propagating with the normal lattice ones but travel with negative group velocities and their wave-packet dispersions opposite to those in the normal lattice modes. Surprisingly, their intensity is high enough to effectively interact with the normal lattice modes under realistic parameters of the lattice and current. This intermode interaction gives rise to new frequency bands where the MSSW intensity is lowered but essentially nonzero (pseudogaps). The pseudogap positions can be shifted by several gigahertz either upwards or downwards with respect to the bandgaps occurring at Brillouin zone edges in the absence of current. The pseudogap shifting depends on the strength of the current and on the lattice magnetization and period.

Anharmonic C-H and C=O Interactions in Peptide and Sugar

C-H and C=O stretching modes are two among many structural and dynamic probes of proteins and peptides in condensed phases. Anharmonic properties of these two modes in peptide and sugar have been examined using a second-order perturbative vibrational approach. High order force constants were obtained and examined to find how crucial they are in determining the degree of mode localization and the nature of mode anharmonicity of the two stretching modes. It is found that the C-H mode is highly localized, and its diagonal anharmonicity is mainly determined by the mode itself. However, the C=O mode is largely delocalized, and the diagonal anharmonicity involves contributions from other modes. The off-diagonal anharmonicity between C-D and C=O modes is found to be negative in deuterated species, differing from those of the non-deuterated ones. It is also found that inter-mode interaction between each of the two modes with low-frequency modes contribute significantly to the off-diagonal anharmonicity. These low-frequency modes give rise to a network of energy relaxation or intramolecular vibrational energy redistribution pathways which can be used to examine temporal behavior of intramolecular vibration energy flow, provided a femtosecond broadband two-dimensional infrared spectroscopy is available.

A model for transition to the mesomorphic state in poly(methyl methacrylate)

The structural instability of a polymer crystal may be due to the involvement of various lattice vibrations in nonlinear resonance. The condition of this involvement is the prevalence of intramode anharmonicity over intermode interaction. A model of phase transitions in polymers of the PMMA type was constructed. Below transition points, coherent states of orientational vibrations of heavy side pendants are formed. The thermodynamic characteristics of these states were found. The structural instability of the polymer lattice is a second-order phase transition.

Decoherence and transfer of quantum states of field modes in a one-dimensional cavity with an oscillating boundary

We study the evolution of Wigner functions of arbitrary initial quantum states of field modes in a one-dimensional ideal cavity, whose boundary performs small harmonic oscillations at the frequency ωW = pω1 (where ω1 is the fundamental field eigenfrequency). Special attention is paid to the case of initial even and odd coherent states, which serve as models of the 'Schrodinger cat states'. We show that the strong intermode interaction (due to the Doppler upshift of the fields reflected from the oscillating mirror) results in the decoherence of initial quantum superpositions in selected modes, even in the absence of any external 'environment'. Different quantitative measures of decoherence are discussed. The analytical solutions obtained show that any initial state of the field goes asymptotically to a highly mixed and moderately squeezed state in the 'principal resonance case' p = 2 and to the vacuum state in the 'semiresonance case' p = 1. It is shown that the decoherence process has several stages. In the first one, the interference between the components of the initial superposition is rapidly destroyed during the time of the primary decoherence, which is inversely proportional to the first power of the initial distance between the components, as opposed to the second power in the case of usual dissipative reservoirs. However, some weak traces of coherence (quantumness of states), such as the regions of negativity of the Wigner function, survive for much longer times, which do not depend on the size of the initial superposition.

Nonlinear oscillations and stability of a charged drop moving relative to a dielectric medium

Nonlinear calculations to within the second order of smallness with respect to the initial deformation of a liquid drop show that a stream of an ideal incompressible dielectric liquid streamlining the charged ideally conducting drop causes interaction between modes both in the first and second orders of smallness. Both the linear and nonlinear interactions of the oscillation modes result in the excitation of modes absent in the spectrum of the initial drop deformation. The relative motion of the drop and the medium leads to broadening of the spectrum of modes excited in the second order of smallness. The presence of the flow streamlining the drop and the intermode interaction result in decreasing the critical magnitudes of the drop charge and the velocity and density of the medium determining drop instability development.

BWR Regional Instability Analysis by TRAC/BF1-ENTRÉE—II: Application to Ringhals Unit-1 Stability Test

Based on the Ringhals unit-1 stability test results, the coupling system TRAC/BF1-ENTRÉE has been benchmarked for predicting decay ratio and limit-cycle amplitude of the regional instability. The core was mapped into fewer CHAN groups based on the first azimuthal mode flux shape and the guidelines to minimize the numerical interregion stabilizing interaction. The system was further applied to detailed phenomenological studies. A symmetric pattern of the first azimuthal mode gave a dynamic boundary condition for ideal out-of-phase flow oscillations and lowers the regional instability threshold. The intermode interaction between the fundamental and first azimuthal modes was demonstrated under postulated large oscillations. Self- and mutual-modal reactivities were evaluated based on the higher modal flux shapes derived by ACCORD-N.