Stochastic resonance in a bistable system with global delay and two noises

The global time delay is introduced into a bistable system driven by two noises and a periodic signal. The signal power amplification factor η is employed to characterise stochastic resonance (SR) of the system. Numerical simulation results indicate the following. (1) For a periodic signal with low frequency (Ω), the typical behaviour of the SR is lowered monotonically by increasing the delay time τ; for moderate Ω, τ enhances the SR behaviour and then weakens it, with a critical value at which the SR is optimum. (2) Multiplicative noise intensity D and additive noise intensity α have different influences on the SR performance, viz D enhances the SR monotonically while α enforces the SR initially and then restrains it; (3) correlation intensity λ between the two noises always weakens the SR behaviour of the system.     

Travelling pulses in reaction-diffusion systems under global constraints

The influence of global inhibition on travelling pulses both in infinite and in finite periodic systems is investigated. Analytic investigation of the modified piecewise linear Rinzel-Keller-Model (RKM) shows a shrinking and slowing down of pulses due to global inhibition. Analytic results obtained are confirmed by numerical simulations. The investigation is completed by numerical simulations of the modified Oregonator model for the light-sensitive BZR. Received: 25 September 1997 / Revised: 9 January 1998 / Accepted: 9 January 1998     

Spin rings in bistable planar semiconductor microcavities

A remarkable feature of exciton-polaritons is the strongly spin-dependent polariton-polariton interaction, which has been predicted to result in the formation of spin rings in real space [Shelykh, Phys. Rev. Lett. 100, 116401 (2008)]. Here we experimentally demonstrate the spin bistability of exciton polaritons in an InGaAs-based semiconductor microcavity under resonant optical pumping. We observe the formation of spin rings whose size can be finely controlled in a spatial scale down to the micrometer range, much smaller than the spot size. Demonstration of optically controlled spin patterns in semiconductors opens way to the realization of spin logic devices and spin memories.     

Switching time in optically bistable injection-locked semiconductor lasers

Switching between states in a dispersive bistable injection-locked slave laser has been theoretically investigated. We show that the switching can be achieved by relatively small and short (≈10-50?ps) variation of the master laser injection power or frequency, which, besides the variation of the slave laser optical power, leads to significant variation of its photon phase (≈5π/6). By using an analytical model, we calculate the switching time dependence on the magnitude of the injection power and the frequency detuning variation.     

Current filamentation instability in laser wakefield accelerators

Experiments using an electron beam produced by laser-wakefield acceleration have shown that varying the overall beam-plasma interaction length results in current filamentation at lengths that exceed the laser depletion length in the plasma. Three-dimensional simulations show this to be a combination of hosing, beam erosion, and filamentation of the decelerated beam. This work suggests the ability to perform scaled experiments of astrophysical instabilities. Additionally, understanding the processes involved with electron beam propagation is essential to the development of wakefield accelerator applications.     

Fluctuations in optical bistable systems

The bistable behaviour of an ensemble of two level atoms driven by a coherent external light field is discussed in the adiabatic limit when the relaxation of the atoms is fast compared to the relaxation of the field. The influence of the different noise sources is discussed separately in a one-dimensional approximation by neglecting the phase fluctuations. In the case where the fluctuations of the driving field play the dominant role, an exact solution of the two-dimensional stationary Fokker-Planck equation is derived describing amplitude as well as phase fluctuations.     

Current transport in bistable granular structures

Electrical breakdown in GaAs Schottky diodes creates a granular filamentary structure of submicron dimensions between the contacts. The filament exhibits very fast (<2 nsec) electrical switching between stable resistance states. Irrespective of the contact metal, the filament becomes superconducting when in the low-resistance state. Electron transport in all the resistance states is intergrain tunneling between metallic inclusions and is governed by an activation energy. Switching is not based on metal transport.     

Current filamentation and macroturbulence in superconductors in rotating magnetic fields

The onset of specific magnetic structures associated with the formation of extended current filaments is observed in YBCO single crystals in a rotating magnetic field. Like current filaments in a plasma, they are unstable and decay into current macrovortices. The appearance of filaments is explained by the formation of closed flux rings along magnetization-reversal fronts and collapse of these rings with formation of Meissner cylinders. The total current along the surface and in the vortex shell of the cylinder (the latter current dominates in high-T _c superconductors) exceeds the critical current in the same volume located far from a filament. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 253–257 (10 February 1997)     

Cavity optimization of optical bistable semiconductor etalon in transmission operation mode

Semiconductor nonlinear Fabry-Perot etalon design is considered. A method of determination of cavity parameters simultaneously satisfying the requirements: low critical switching intensity, high output signal and high contrast between two stable state, is proposed. The results obtained may be used for construction of optical bistable elements in the transmission operation mode.     

Stochastic resonance in underdamped,bistable systems

We carry out a detailed numerical investigation of stochastic resonance in underdamped systems in the nonperturbative regime. We point out that an important distinction between stochastic resonance in overdamped and underdamped systems lies in the lack of dependence of the amplitude of the noise-averaged trajectory on the noise strength, in the latter case. We provide qualitative explanations for the observed behaviour and show that signatures such as the initial decay and long-time oscillatory behaviour of the temporal correlation function and peaks in the noise and phase averaged power spectral density, clearly indicate the manifestation of resonant behaviour in noisy, underdamped bistable systems in the weak to moderate noise regime.     

Spontaneous switching in cavityless optically bistable systems

Relaxation cavityless optical bistability is studied when a semiconductor is exposed to light quanta with an energy exceeding the transition energy and the free-electron relaxation time depends on temperature. The numerical analysis of dynamic equations for temperature and free-electron density demonstrates that 11 regimes of light-semiconductor interaction are possible. The regimes are classified in terms of the features of temperature-intensity characteristic curves, namely, by the number and arrangement of instability and quasistability segments and by the presence or absence of bistability. It is found that both the upper and the lower branch of a bistable characteristic curve may be unstable, which results in spontaneous switchings between the respective states. Similarly, if the upper state is unstable and the lower state is stable, a small perturbation of the parameters (free-electron temperature and density) near the former causes the transition to the oscillatory regime. If the upper state is unstable and a small perturbation of the lower (stable) state excites a damped oscillation, spontaneous switchings result if the fluctuating temperature exceeds some critical value for the lower state.     

Long switching times in absorptive bistable systems

We study the switching characteristics of a Fabry-Pérot cavity containing a saturable absorber and driven by an external laser. It is shown how long switching times can arise in such a system and an expression is given for the switching time.     

Mechanical systems with nonlinear constraints

A geometrical formalism for nonlinear nonholonomic Lagrangian systems is developed. The solution of the constrained problem is discussed by using almost product structures along the constraint submanifold. Constrained systems with ideal constraints are also considered, and Chetaev conditions are given in geometrical terms. A Noether theorem is also proved.     

Higher-order mechanical systems with constraints

We extend Weber's approach for mechanical systems with constraints in terms of differential-geometric structures underlying higher-order tangent bundles.     

Current filamentation in GaAs/AlGaAs heterojunctions preceding quantum-Hall-effect breakdown

Self-induced filamentation of the current near the edges of a sample in the Hall geometry has been observed in the quantum-Hall-effect regime in a two-dimensional electronic system arising near a GaAs/AlGaAs heterojunction. If in the case of integer values of the filling factor ν averaged over the sample the currents flowing along opposite edges are approximately the same, then away from such a value within the quantum plateau the current is increasingly concentrated near that edge of the sample where the local value of ν is closer to being an integer. When the direction of the magnetic field or of the current changes, the filament switches to the opposite edge of the sample. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 9, 700–704 (10 November 1998)