Low Coupling Strength Research Articles

Equilibrium bifurcations and chaotic transitions in coupled microlaser lattices

Analytic and simulation studies for the steady-state equilibria and bifurcations of coupled microlaser arrays are described. Lateral cavity interactions affect the gain in each cavity, leading to active photonic lattice behavior, equivalent to a nonlinear coupled oscillator lattice. The coupled-cavity rate equations are employed to follow the coherent photon and carrier population in each lattice site. Fixed-point-type steady states, of constant lattice phase shift, result for low coupling strengths; the radiation envelope for these states conforms with a periodic Bloch state over the array. Bifurcations to limit cycles of increasing complexity occur at higher coupling via period doubling sequences. The associated spatial patterns of photon and carrier lattice distribution resemble photonic convection cells. Limit cycles of different periods, emanating mathematically from different original fixed points, coexist at high strengths, each one accessible from different initial conditions. The multiplicity of possible limit cycles in systems with many degrees of freedom (number of lattice sites) combined with changes in their accessibility from initial conditions offers new insights to chaotic transitions, compared to low dimensionality paradigms.

Photoionization dynamics from [formula omitted]: a one-electron model

Quantum dynamics of photoionization from Cl 2 - is investigated by constructing a purely one-electron model Hamiltonian of the anion calibrated with the help of ab initio theoretical calculations at the MRSD-CI level. A mean-field time-dependent Fourier grid Hamiltonian method is used to investigate the modulation of the dynamics of photoionization from Cl 2 - caused by the coupling of electronic and nuclear motions. Vibrational quenching at low coupling strength is predicted to turn into vibrational enhancement of ionization at higher strengths of coupling.

Enhancement of neural synchrony by time delay.

In a network of neuronal oscillators with time-delayed coupling, we uncover a phenomenon of enhancement of neural synchrony by time delay: a stable synchronized state exists at low coupling strengths for significant time delays. By formulating a master stability equation for time-delayed networks of Hindmarsh-Rose neurons, we show that there is always an extended region of stable synchronous activity corresponding to low coupling strengths. Such synchrony could be achieved in the undelayed system only by much higher coupling strengths. This phenomenon of enhanced neural synchrony by delay has important implications, in particular, in understanding synchronization of distant neurons and information processing in the brain.

Effect of Surface Modification on Back Electron Transfer Dynamics of Dibromo Fluorescein Sensitized TiO2 Nanoparticles

Electron injection and back electron transfer (BET) dynamics have been carried out for dibromo fluorescein (DBF) sensitized TiO2 nanoparticles capped (modified) with sodium dodecyl benzene sulfonate using transient absorption techniques in picosecond and microsecond time domain. BET dynamics have been compared with bare (unmodified) nanoparticles for the same DBF/TiO2 system. It has been observed that BET reaction is slow on the modified surface compared to a bare surface in earlier time domain (picosecond). This observation has been explained by the fact that on surface modification the energy levels of the semiconductor nanoparticles are pushed up in energy. As a result, the free energy of reaction (-deltaG zero) for BET reaction of a dye/SM-TiO2 system increases as compared to the dye/bare TiO2 system. High exoergic BET reaction in dye-sensitized TiO2 nanoparticles surfaces fall in the Marcus inverted regime, so with increasing free energy of reaction, BET rate decreases on the modified surface. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond). In microsecond time domain BET reaction is faster on the modified surface as compared to on the bare surface. Modification of this surface reduces the density of deep trap states. Recombination dynamics between deep-trapped electron and parent cation is slow due to low coupling strength of BET reaction. As the density of deep-trapped electrons is high in bare particles, BET reaction is slow in longer time domain.

Self-consistent Overhauser model for the pair distribution function of an electron gas in dimensionalitiesD=3andD=2

We present self-consistent calculations of the spin-averaged pair distribution function $g(r)$ for a homogeneous electron gas in the paramagnetic state in both three and two dimensions, based on an extension of a model that was originally proposed by Overhauser [Can. J. Phys. 73, 683 (1995)] and further evaluated by Gori-Giorgi and Perdew [Phys. Rev. B 64, 155102 (2001)]. The model involves the solution of a two-electron scattering problem via an effective Coulombic potential, which we determine within a self-consistent Hartree approximation. We find numerical results for $g(r)$ that are in excellent agreement with quantum Monte Carlo data at low and intermediate coupling strength ${r}_{s},$ extending up to ${r}_{s}\ensuremath{\approx}10$ in dimensionality $D=3.$ However, the Hartree approximation does not properly account for the emergence of a first-neighbor peak at stronger coupling, such as at ${r}_{s}=5$ in $D=2,$ and has limited accuracy in regard to the spin-resolved components ${g}_{\ensuremath{\uparrow}\ensuremath{\uparrow}}(r)$ and ${g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}(r).$ We also report calculations of the electron-electron s-wave scattering length, to test an analytical expression proposed by Overhauser in $D=3$ and to present new results in $D=2$ at moderate coupling strength. Finally, we indicate how this approach can be extended to evaluate the pair distribution functions in inhomogeneous electron systems and hence to obtain improved exchange-correlation energy functionals.

NOISE-ENHANCED PROPAGATION OF BICHROMATIC SIGNALS

We examine the influence of noise on the propagation of harmonic signals with two frequencies through discrete bistable media. We show that random fluctuations enhance propagation of this kind of signals for low coupling strengths, similarly to what happens with purely monochromatic signals. As a more relevant finding, we observe that the frequency being propagated with better efficiency can be selected by tuning the intensity of the noise, in such a way that for large noises the highest frequency is transmitted better than the lower one, whereas for small noises the reverse holds. Such a noise-induced frequency selection can be expected to exist for general multifrequency harmonic signals.

Synchronization of Oscillations and Propagation of Excitations in Circular and Linear Arrays of Coupled CSTRs

Synchronizations of oscillatory regimes of the Belousov−Zhabotinskii (BZ) reaction in a circular array of three identical CSTRs coupled via symmetric passive diffusion/convection mass transfer were studied experimentally. Stability of symmetric and asymmetric phase-shifted oscillatory regimes with respect to variations of the coupling strength among the reaction cells was examined. The all-in-phase regime was found to be the only regime stable over the entire range of coupling strength values. Phase-shifted oscillatory regimes were found to be stable only within a narrow interval of very low coupling strength values. Spontaneous transitions of the phase-shifted regimes to the synchronized mode due to stochastic fluctuations of the coupling strength were observed. Numerical simulations with the four-variable Oregonator based model of the BZ reaction qualitatively confirmed the experimental findings. Propagation of an excitable response to periodic pulsed stimulations in a linear three-array of coupled chem...

High-Quality Ni-Fe/Cu Multilayer Films with Antiferromagnetic Coupling

We report structural and magnetic properties of Ni83Fe17/Cu multilayer films with various buffer layer and sublayer thicknesses of copper dcu and Permalloy dp y deposited by face-to-face sputtering. The following features prove a good quality of our films: a well-layered structure, complete antiferromagnetic coupling with a low coupling strength (2 x 10 -5 J/m2 for dcu = 1 nm and 10 -5 J/m2 for dc = 2.1 nm) and a low coercive field which make them attractive for possible applications as giant magnetoresistance sensors.

Quantum mechanical and semiclassical dynamics at a conical intersection

We present simulations of wave-packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two-dimensional problem and for one-dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.

Coupled chaotic states and apparent noise in experiment and model

We present an experimental and model study of the effect of mass coupling of two similar chaotic states in the Belousov–Zhabotinskii (BZ) reaction. At high coupling strengths the coupled chaotic states become synchronized as shown by a high correlation coefficient. When the coupling strength is decreased, the coupled system passes through a symmetry–breaking transition from synchronized to asynchroneous chaos. At the transition point the direct experimental evaluation of the maximum Lyapunov exponent of the single chaotic system is possible from the coupling strength. At very low coupling strengths the correlation approaches zero. The differences and sums of the two chaotic time series in each reactor are also investigated. At high and moderate coupling strengths, the differences and the sums are verified to be deterministically chaotic on the basis of their fractal dimensionalities, for example. However, for weakly coupled (and uncoupled) chaotic states our analysis with state-of-the-art methods shows that the sum and differences behave as if they are stochastic, although the time series of individual reactions always remain deterministically chaotic. The reason for this apparent ‘‘statistical’’ behavior at low (and zero) coupling strengths is the limited number of the experimental (∼16 000 data points) and numerical data points available for analysis for large Hausdorff dimensions (DH≳3.6). Hyperchaos could not be detected in the coupled chaotic BZ systems.

Two coupled Ising planes: Phase diagram and interplanar force

The statistical mechanics of two Ising ferromagnetic planes coupled by a local interlayer two-spin interaction have been studied by means of a variety of calculational methods, including different mean-field approximations, Migdal-Kadanoff-type renormalization-group techniques, scaling theory, as well as numerical Monte Carlo simulation techniques. The phase diagram has been derived as a function of the interlayer coupling strength. Furthermore, various thermodynamic variables have been determined, including the interlayer correlation function, which is proportional to the interplanar force. It is found that a Migdal-Kadanoff renormalization with a decimation procedure which involves the interlayer coupling in an appropriate fashion provides an accurate description of the phase diagram and that a mean-field description provides a good description of the phase diagram and the interplanar force, except for very low interlayer coupling strengths.

Continuous Coupling of Chaotic and Periodic States of Chemical Oscillators With and Without Time Delay

AbstractWe present continuous coupling experiments and calculations of two mutually flow‐rate‐coupled Belousov‐Zhabotinsky (BZ) oscillators. The Ce4+‐concentration of one BZ‐reactor controls the flow rate into the other BZ‐reactor and vice versa. Flow‐rate control is proportional to the difference between the (time delayed) Ce4+‐and an average Ce4+‐concentration. When both period one (P1) BZ oscillators are weakly coupled, the P1 oscillations are retained in both oscillators whose periods increase with increasing coupling strength. At high coupling strengths statistically aperiodic oscillations are produced. The introduction of time delayed coupling does not reduce the noise in the strongly coupled system. We distinguish between statistical aperiodicity due to experimental noise and deterministic chaos due to the system's nonlinear dynamics. At constant coupling strength a rising delay time produces a “sawtooth” like behaviour of the P1 periods. Mutual flow‐rate‐coupling of two chaotic states without time delay retains chaos in both reactors at low coupling strengths. When the coupling strength increases the Hausdorff dimension passes through a maximum (≈︁ 2.5). At very high coupling strengths, a transition from chaos to periodic states takes place.

The spectral function of a one-dimensional Holstein polaron

The spectral function for a Holstein polaron in one dimension is calculated using exact diagonalization techniques, as a function of Einstein phonon frequency and electron-phonon coupling strength. We also use the self-consistent Migdal approximation to calculate the same spectrum; we find quantitative agreement for all frequencies at low coupling strengths. The ground state energy and quasiparticle residue evolve smoothly from band electron like to small polaron like. We compare with Migdal and small polaron approximations and find that neither is in quantitative agreement with the exact result at intermediate coupling strengths.

Tapered, resonant DBR transmission filters

Wavelength-tunable optical filters, in the form of resonant distributed Bragg reflector laser filters, have been fabricated and evaluated. The devices are specially designed as transmission filters and have a novel structure, using low coupling strength in the grating and an up-tapered active section, in order to improve the crosstalk performance and the saturation properties. Fabricated devices have 6-8-dB dynamic range and would give less than 1-dB crosstalk penalty in four-channel systems at 13-dB chip gain. System experiments with two 2-Gb/s channels are reported. >

Mutually coupled oscillators with time delay

We present a system of two identical chemical oscillators which are coupled by means of their flow rates. The individual flow rate of one oscillator is controlled by the output of the other oscillator and vice versa. A delay time is applied between the output of one oscillator and the input to the other one. This delay has been investigated both theoretically and experimentally for the minimal bromate oscillator. By variation of the delay time both oscillators dramatically change their periods and amplitudes. At lower coupling strengths, the periods of both oscillators increase and decrease in a sawtooth fashion with increasing delay times. At high coupling strengths multimode patterns were obtained at high delay times. No quasiperiodic or aperiodic behavior was observed in this coupled system. The experimental results are in general agreement with those calculated with the Noyes-Field-Thompson mechanism. 38 refs., 8 figs.

A coupled pacemaker-slave model for the insect photoperiodic clock: interpretation of ovarian diapause data in Drosophila melanogaster.

A coupled circadian oscillator model for the insect photoperiodic clock is described which consists of a hierarchically arranged pacemaker and slave. The pacemaker is self-sustained, temperature compensated, and entrainable by the light cycle; the slave is a damping oscillation receiving entrainment from two sources, from the pacemaker via a coupling factor, and also directly from the light. The damping slave oscillation is seen as the "photoperiodic oscillator", equivalent to that proposed earlier by Lewis and Saunders (1987). The present simulations describe the effect of the strength of the coupling factor between hypothetical short- and long-period pacemaker oscillations (modelled on the "clock" mutants perS and perL2 in Drosophila melanogaster) and a slave oscillation with a period of about 24 hours. The output is presented in terms of photoperiodic response curves and Nanda-Hamner, or resonance, plots. With a high coupling strength, the pacemakers strongly entrain the slave, but with a low coupling strength the slave's properties are more evident. The model is presented as a possible explanation for recent ovarian diapause data in D. melanogaster "clock" mutants (Saunders 1990), but also as a more general model for the role of the insect circadian system in seasonal time measurement.

Effects of coupling nonlinear systems with complex dynamics

Dynamic behavior of coupled cells, each of which is exhibiting a complex pattern, has been studied as a function of a coupling parameter for a simple model system. Coupling of cells with different dynamics for different cases show a generalized feature, namely, presence of a hyperchaotic regime at low coupling strengths followed by a periodic window at intermediate values followed by synchronization in a chaotic state. The interesting observation of complete synchronization of individual cells even when the coupled dynamics show chaos is of considerable significance.

Modulational instability in nonlinear periodic structures: Implications for ‘‘gap solitons’’

A nonlinear periodic structure can convert an input continuous wave beam into a train of pulses. This modulational instability implies that the nonlinear spatial resonance (‘‘gap solitons’’) of distributed feedback structures are generally unstable. Stability is assured only for low coupling strengths or large detunings from the Bragg condition.

Series compensation schemes reducing the potential of subsynchronous resonance

New series compensation schemes that reduce the potential of synchronous resonance problems in series compensated electrical power systems are presented. The schemes are phase-wise compensation schemes with three different combinations of inductive and capacitive elements. The combinations chosen have frequency characteristics that give equal reactances at the power frequency and unequal reactances at the other frequencies. This implies that the subsynchronous oscillations drive unsymmetrical three-phase currents. The developed electric torque, therefore, will be lower than what it would be if the three-phase currents were symmetrical. This results in a lower coupling strength between the mechanical and electrical systems at asynchronous oscillations. Therefore, the energy exchange between the electrical and mechanical systems at subsynchronous oscillations will be suppressed, avoiding the build-up of torsional stresses on the generator shaft systems under subsynchronous resonance conditions. The validity and effectiveness of the schemes have been demonstrated on the Second IEEE Benchmark Model, by means of time simulation analysis using the EMTP program. >

Critical current density of YBa 2Cu 3O 7-x sintered pellets under transverse magnetic field

Superconducting YBa 2Cu 3O 7-x pellets, having transition temperatures between 85 and 90K have been prepared under two different sintering conditions, and influence of magnetic field on critical current densities of these pellets measured at 78K have been reported here. A strong field dependence (H<1KOe) of J c has been observed. Results have been explained assuming the presence of a large number of intergranular links with very low coupling strength.