Characteristics of critical amplitude of a sinusoidal stimulus in a model neuron

The characteristics of the critical amplitude of a sinusoidal stimulus in a model neuron, Morris－Lecar model, are investigated numerically. It is important in the study of stochastic resonance to determine whether a periodic stimulus is subthreshold or not. The critical amplitude as a function of the stimulus frequency is not a constant, but a curve, which is the boundary between subthreshold and suprathreshold stimulation. It has been considered that this curve is U-shaped in the previous investigations, and this has been accepted as a universal phenomenon. Nevertheless, we think that it is only true for a type of neuron: namely, resonators. Actually, there exists another type of neuron, integrators, which can undergo a saddle-node on invariant circle bifurcation from the rest state to the firing state. For the latter we find that the critical amplitude increases monotonically as the frequency of sinusoidal stimulus is increased. This is shown by way of the Morris－Lecar model. As a consequence, the critical amplitude curve is studied further, and the dynamical mechanisms underlying the change in critical amplitude curve are uncovered. The results of this paper can provide a reference to choose the subthreshold periodic stimulus.     

Microstructure of a Spark Discharge in Air in a Point–Plane Gap

A microchannel structure of a spark discharge initiated in atmospheric-pressure air in a point–plane gap has been discovered by shadow photography. The structure has been observed since the onset of the discharge glow. The evolution of the microstructure over times from several nanoseconds to several tens of nanoseconds has been traced. Specifically, the development of microchannels from the point into the discharge gap, expansion of the microchannels, and their interaction have been observed. A correlation has been found between the microstructure of the spark channel in the discharge gap and its autograph on the surface of the plane electrode.     

Theory of sound field in a room

In the normal-mode theory of Morse, it gives a series of normal modes as the solution of forced vibration in a room. But actually there is always the direct radiation besides the normal modes which represent the reverbrant sound field only. The reason is that the normal modes were assumed only in the source, and naturally normal modes only are obtained in the solution. A theory of double source is proposed, that the sound source is both the source of the direct radiation as if in free space before the boundary surfaces were reached by the direct radiation, and after the first reflection from the boundary surfaces, the source of the reflected wavelets, randomly distributed both in space an in time on the boundary surfaces that build up the normal modes after further reflections. The wave equation is formed accordingly, and the solution of the wave equation, the sound field in a room, contains explicitly both the direct radiation and the reverberant sound formed of normal modes. The approximate mean square sound pressure is found to be the dircet sound determined by the sound power of the source,and reverberant sound determined by the sound power reduced by a factor of π/2, different slightly from the result obtained from energy consideration, if the source is pure tone. There is essentially no difference for a source of band noise.     

Nonlinear Dynamics of a Sliding Chain in a Periodic Potential

Nonlinear dynamics of the sliding process of a chain driven with a constant velocity at one end in a periodic substrate potential is investigated. The driven chain exhibits distinctly different dynamical characteristics at different velocities. In the low velocity region, the chain moves in a stick-slip manner. When the driving velocity is increased, the stick-slip behaviour is replaced by complicated and regular oscillatory motions. The dependence of the dynamics on the coupling strength is studied and the step-like behaviour is found, where different steps correspond to different dynamical phases.     

Phase Dependence of Fluorescence Spectrum of a Two-Level Atom in a Trichromatic Field

We examine the phase-dependent effects in resonance fluorescence of a two-level atom driven by a trichromatic modulated field. It is shown that the fluorescence spectrum depends crucially on the sum of relative phases of the sideband components compared to the central component, not simply on the respective phases. The appearance or disappearance of the central peak and the selective elimination of the sideband peaks are achieved simply by varying the sum phase. Once the sum phase is fixed, the spectrum keeps its features unchanged regardless of the respective relative phases.     

Behaviuour of Polariton in a Simple Model

Behaviour of a simple model polariton system is restudied.The distribution of phonons in polartion never is sub-Poisson given by computation of the thermal counterpart of the Q parameter and the matrix m^(3).The polariton complex as a whole shows non-classical behaviour below a threshold temperature,which depends on the photon-phonon coupling strength,when the distribution of phonons is always classical at any temperature.     

Is there a spatial correlation in the distribution of adsorbed atoms in the cages of a zeolite?

Monte Carlo simulations of the cage-to-cage jumps of Xe atoms in a crystalline zeolite using the experimentally observed molecular rate constants for cage-to-cage jumps were carried out to determine if there is a systematic spatial correlation of the distributions of Xe atoms among the cages at equilibrium. The neighbours of cages having an Xe occupancy that is less than the average occupancy are found to have distributions that are skewed toward higher occupancy compared with the overall distribution. On the other hand, the neighbours of cages having an Xe occupancy that is greater than the average occupancy are found to have distributions that are skewed toward lower occupancy than the overall distribution.     

Spectra of spin-wave excitations of a 71° domain boundary in a cubic ferromagnet

Features of the long-wavelength part of the spectrum of spin waves localized on a 71° domain boundary of a cubic ferromagnet are examined on the basis of a qualitative analysis and simultaneous numerical solution of the Landau-Lifshitz equations and the equation of magnetostatics. Fiz. Tverd. Tela (St. Petersburg) 41, 2183–2186 (December 1999)     

Radiation Rate of a Two-Level Atom in a Spacetime with a Reflecting Boundary

We study a two-level atom in interaction with a real massless scalar quantum field in a spacetime with a reflecting boundary. We calculate the rate of change of the atomic energy for the atom. The presence of the boundary modifies the quantum fluctuations of the scalar field, which in turn modifies the rate of change of the atomic energy. It is found that the modifications induced by the presence of a boundary make the spontaneous radiation rate of an excited atom to oscillate near the boundary and this oscillatory behaviour may offer a possible opportunity for experimental tests for geometrical （boundary） effects in flat spacetime.     

Explosive synchronization in a mobile network in the presence of a positive feedback mechanism

Synchronization is a process that describes the coherent dynamics of a large ensemble of interacting units.The study of explosive synchronization transition attracts considerable attention.Here,I report the explosive transition within the framework of a mobile network,while each oscillator is controlled by global-order parameters of the system.Using numerical simulation,I find that the explosive synchronization(ES)transition behavior can be controlled by simply adjusting the fraction of controlled oscillators.The influences of some parameters on explosive synchronization are studied.Moreover,due to the presence of the positive feedback mechanism,I prevent the occurrence of the synchronization of continuous-phase transition and make phase transition of the system a first-order phase transition accompanied by a hysteresis loop.     

Numerical investigation of separation-induced transition in a lowpressure turbine cascade in a low-disturbance environment

In this paper,the separation-induced transition in an LPT(low-pressure turbine)cascade is investigated at low Reynolds number with DNS(direct numerical simulation).The transition process is accurately predicted giving good agreements between the DNS and experimental results.To illustrate the secondary instability of separation-induced transition in a low-disturbance environment,the results are comprehensively analyzed in both Fourier space and physical space.It is illustrated that the effect of hyperbolic instability dominates around the saddle point of hyperbolic streamlines.This instability mechanism is responsible for the emergence of the streamwise vortices in the braid region.Elongated and intensified because of the“stretching”effect of the background flow,these vortices become the most noticeable characteristic of the flow field.Fundamental modes of small spanwise wavelength are excited in the braid region,so as some low-frequency modes.The elliptical instability plays a minor role than hyperbolic instability.It is also observed that the fundamental mode with a larger spanwise wavelength is unstable in the vortex core which is associated with the deformation of the vortex core via elliptical instability.There is no convincing evidence for the existence of subharmonic instability.     

Probe spectrum of a four-level atom in a double-band photonic crystal

In this paper, the probe absorption spectrum of an atom in a double-band photonic crystal have been studied. In the modes, we assume that one of the two atomic transitions in a Λ-type atomic system is interacting with free vacuum modes, and another transition is interacting with free vacuum modes, isotropic photonic band gap (PBG) modes and anisotropic PBG modes, separately. The effects of the fine structure of the atomic lower levels on the probe absorption spectrum are investigated in detail in the three cases. The most interesting thing is that the two (four) transparencies at one (two) probe absorption peak(s), caused by the fine structure of the lower levels of an atom, are predicted in the case of isotropic PBG modes.     

Ground State of a Polaron in a Symmetric Triangular Quantum Well

We study the effects of electron-phonon interaction on the electron ground state in a symmetric triangular quantum well, and calculate the ground state energy of an electron in the GaAs/Al0.96Ga0.04As triangular quantum well including the effects of the interaction between electrons and confined LO phonons by using a modified Lee-Low-Pines variational method. The electron wavefunction in the triangular well is chosen as the Airy function. The numerical results are given and discussed.     

Absorption Spectra of a Three-Level Atom Embedded in a PBG Reservoir

We introduce the ‘decay rate＇ terms into the density matrix equations of an atom embedded in a photonic band gap （PBG） reservoir successfully. By utilizing the master equations, the probe absorption spectra and the refractivity properties of a three-level atom in the PBG reservoir are obtained. The interaction between the atom and the PBG reservoir as well as the effects of the quantum interference on the absorption of the atom has also been taken into account. It is interesting that two different types of the anomalous dispersion relations of refractivity are exhibited in one dispersion line. The methodology used here can be applied to theoretical investigation of quantum interference effects of other atomic models embedded in a PBG reservoir.     

Ballistic Diffusion of a Charged Particle in a Blackbody Radiation Field

The generalized Langevin equation is used to describe the motion of a charged particle interacting with a blackbody radiation field via dipole coupling. The exact expressions for the mean-square displacement and velocity of such a particle are obtained, which show a ballistic diffusion and a modified Kubo fluctuation-dissipation relation. In particular, a velocity-dependent coupling or force can induce this novel phenomenon.     

Active control of intermittent noise in a room

I.IntroductionThereverberantnoisesmaybereducedwithanactivenoisecontrol(ANC)systcmformedbyacornermicrophonc-amplifier-1oudspcakersystemina.oom.lllThismeth-odbascdonthenormalmodccancelingprincip1chasalreadybeenclarifiedbothinthco-ryandexperimcnts,andthclimitationandpotcntialityofitsapp1icationalsodiscussedinapreviouspaper.Thediscussionsinthepastlimitedmost1ytothesteadyconditionofnoiscfield,andthisisextendedtothetransientcondihoninthcprcsentwork,viz.thein-llucnceofthecornerANCsystemwhenthenois…     

Eigenstates of Laser-Assisted Scattering in a Noninertial Frame

In a rotating noninertial frame, we investigate the eigenstates of the time-dependent problem for electronnucleus scattering assisted by a circularly polarized laser field. Numerical results of probability distribution, quantum potential, and current density are discussed. An approximate expression of scattering cross section for low laser frequency is given.     

Investigation of switching-window in a gain-transparent UNI configuration

Switching-window in an interferometric configuration of a gain-transparent ultrafast nonlinear interferometer (gt-UNI) is investigated numerically. The phase change is observed in detail. To assess the performance of switching window, the integrated contrast ratio (ICR) is introduced. For the data pulse with the bit width of 6.25 ps and with the energy of 60 pJ, respectively, the ICRs in different situations are simulated using modified nonlinear Schrodinger equation (MNLSE) considering of the carrier depletion pulsation (CDP), carrier heating (CH), and spectral-hole burning (SHB). The results show that the maximum ICR is located at the optimum position of the semiconductor optical amplifier (SOA) that is determined by the width and energy of the control pulse.     

Linear Stability of Flows in a Squeeze Film

We study linear stability of viscous flows in a squeeze lubrication film, in which the flow varies slowly in space and time, between two parallel plates moving normal to each other with a slow constant speed, generalizing the inviscid results of Aristov and Gitman [J. Fluid Mech. 464 (2002) 209]. The temporal evolution of twodimensional disturbances for this physical situation, including the asymptotic behaviour of a long term or the transient behaviour of some time interval, is obtained by the construction of a low-dimensional Galerkin method.It is found that the wall boundaries typically play dual roles of stabilizer and destabilizer. They constrain the development of disturbances and have stabilizing influences. However, they give rise to velocity shear, which is diffused by viscosity and thereby tends to destabilize the flow.