外界电场激励下的耦合FitzHugh-Nagumo神经元系统的放电节律研究

研究了单向耦合连接的两个FitzHugh-Nagumo神经元系统的动力学行为.随外激频率的变化,系统表现出p:q锁相（一种周期振荡,q周期刺激产生p周期动作电位）,且锁相是否发生与放电状态有关.研究表明外激频率和耦合强度都可以引起系统峰峰间期（interspike Interval,ISI）分岔,而外激频率对系统放电节律的影响更为明显,研究还发现混沌态是其他放电状态的过渡态. 关键词： FHN神经元 耦合 动力学行为     

Two-photon excitation of atomic oxygen using a Raman-shifted ArF-excimer laser

A novel method for exciting the two-photon 2p ³ 3p ³ P 2p ^{4 3} P atomic oxygen transition is reported. The transition energy is provided by the combined photon energies of the second and third Stokes orders of a deuterium-filled Raman shifter pumped by an ArF-excimer laser. The ArF-excimer-pumped Raman shifter performance is characterized and the approach is demonstrated in an atomic oxygen population created by molecular oxygen absorption and predissociation in room air.     

Structural Properties of Additive Nano/Microcellular Automata

A detailed structural analysis of 2D patterns generated by one‐dimensional M‐state additive cellular automata (ACA), where M is a prime (p) to a non‐negative power (t), is performed by using the transition matrix (TM), the barycentric fixed‐mass (BFM) method, and the small‐angle scattering (SAS) technique. The BFM method shows that, for M‐state ACA, the subsets of particular states are multifractals. The fractal dimensions of ACA of a given transition rule are shown to be equal at constant p and arbitrary t, and there is a prime p at which the fractal dimensions of different ACA are equal. The SAS technique is shown to be able to differentiate between patterns of such ACA. In addition, SAS can also provide complementary information about the overall size, number of elements, and number of rows generated by ACA. In the case of monofractals, SAS can recover also the scaling factor, which is shown to be, for the ‐state ACA, the inverse of p. The above findings on the 2‐ and 3‐state Rule 90 and Rule 150 are illustrated as idealized models with possible applications toward assembly of nanoparticles into programmable patterns or in modeling biological systems such as fibroblast aggregation, the growth of bacterial colonies, neuronal maps, and virus spreading.     

Metastability in the Potts model on the Cayley tree

Metastability in the ferromagneticp-state Potts model defined on the Cayley tree is discussed. It is shown that the sign of the boundary fieldH _s determines the order of the transition as well as the stability of the low-temperature phase. Lowering the temperature withH _s >0, a system withp<2 (p>2) will display a second (first)-order transition to a metastable (stable) phase. ForH _s >0 a second (first)-order transition to a metastable (stable) phase occurs ifp>2 (p<2). In this case the system also has a residual entropy which is negative forp<2.     

Piezoelectric properties of non-uniform polymeric structures under static load

ABSTRACT

Piezo-electrical properties of structures containing ‘soft’ and ‘hard’ dielectric layers with charge stored on the interfaces are described in the paper. The piezo-activation process of structures containing layers with gas voids by partial discharges is described. The influence of the mechanical properties of the ‘soft’ layer on the piezoelectric parameter d₃₃ value and its dependence on the static pressure p are also discussed. It was found experimentally, that for the fibrous type of ‘soft’ dielectric layer, the dependence of the piezoelectric parameter d₃₃ (p) can be described by the function d₃₃ ÷ p^?n, where n ≤ 1.     

Phase Transition between Synchronous and Asynchronous Updating Algorithms

We update a one-dimensional chain of Ising spins of length L with algorithms which are parameterized by the probability p for a certain site to get updated in one time step. The result of the update event itself is determined by the energy change due to the local change in the configuration. In this way we interpolate between the Metropolis algorithm at zero temperature when p is of the order of 1/L and L is large, and a synchronous deterministic updating procedure for p=1. As a function of p we observe a phase transition between the stationary states to which the algorithm drives the system. These are non-absorbing stationary states with antiferromagnetic domains for p>p _c , and absorbing states with ferromagnetic domains for p≤p _c . This means that above this transition the stationary states have lost any remnants of the ferromagnetic Ising interaction. A measurement of the critical exponents shows that this transition belongs to the universality class of parity conservation.     

Magnetic effects on the dynamics of organic triplets in the level anti-crossing region

A fast magnetic field sweep (2.0 × 10⁵ T s^?1) is applied to probe the spin dynamics in the level anti-crossing (LAC) region of the photo-excited triplet states of pentacene-h ₁₄ in p-terphenyl (PHPT) and pentacene-d ₁₄ in p-terphenyl (PDPT) systems. From the pulsed zero-field FID (ZF FID) EPR measurements of the triplet signal at room temperature, the crossing is found to be a non-adiabatic passage at such a high jumping rate. Detailed examination of the initial population of the photo-excited triplet states of these two systems indicates that the non-adiabatic state mixing is the dominant mechanism for the polarization transfer (electron-nuclear spins) in the LAC region. The time dependent Schrödinger equation is applied to calculate the non-adiabatic transition probabilities, and to simulate the observed triplet population profiles with fast field sweeping.     

Ab initio cluster calculations of Co3+ spin states in RBaCo2O5.5 (R=Ho, Gd)

Two types of oxygen-deficient perovskites RBaCo₂O_5.5(R=Ho,Gd) related to the “122” type structure (a _p × 2a _p × 2a _p ) have been studied on the basis of ab initio cluster calculations. We consider the peculiar behavior of the trivalent ions Co³⁺(3d ⁶) in either octahedral or pyramidal oxygen coordinations, which is related to a structural first-order phase transition in both compounds. Relative energy positions of low spin (LS, S = 0), intermediate spin (IS, S = 1) and high spin (HS, S = 2) electron configurations are calculated for the low-and high-temperature lattice structures of RBaCo₂O_5.5. A combined analysis of the calculated results and experimental structural data leads to a simple model that captures the most prominent features of the phase transition common to both compounds.     

Thermal analysis of specific heat measurements in glassy Se80−xTe20Sbx alloys in glass transition region

In the present report, we have done specific heat measurements in glassy Se_80?xTe₂₀Sb_x (0 ≤ x ≤ 15) alloys in glass transition region. Differential scanning calorimetry (DSC) technique is used for this purpose. We have observed a tremendously huge increase in the specific heat (C_p) values at the glass transition temperature. The thermal analysis shows that the values of C_p below glass transition temperature and the difference of C_p values before and after glass transition (?C_p) are highly composition-dependent.     

Phase diagram of the Mott transition in a two-band Hubbard model in infinite dimensions

The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the d-orbital, the charge-transfer energy between the d- and p-orbitals and the hopping integrals between p-d, d-d and p-p orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method. Received 5 October 2000 and Received in final form 8 December 2000     

Schloegl’s Second Model for Autocatalysis on a Cubic Lattice: Mean-Field-Type Discrete Reaction-Diffusion Equation Analysis

Schloegl’s second model for autocatalysis on a hypercubic lattice of dimension d≥2 involves: (i) spontaneous annihilation of particles at lattice sites with rate p; and (ii) autocatalytic creation of particles at vacant sites at a rate proportional to the number of diagonal pairs of particles on neighboring sites. Kinetic Monte Carlo simulations for a d=3 cubic lattice reveal a discontinuous transition from a populated state to a vacuum state as p increases above p=p _e . However, stationary points, p=p _eq (≤p _e ), for planar interfaces separating these states depend on interface orientation. Our focus is on analysis of interface dynamics via discrete reaction-diffusion equations (dRDE’s) obtained from mean-field type approximations to the exact master equations for spatially inhomogeneous states. These dRDE can display propagation failure absent due to fluctuations in the stochastic model. However, accounting for this anomaly, dRDE analysis elucidates exact behavior with quantitative accuracy for higher-level approximations.     

Order-parameter fluctuations (OPF) in spin glasses: Monte Carlo simulations and exact results for small sizes

The use of parameters measuring order-parameter fluctuations (OPF) has been encouraged by the recent results reported in referenece [2,3] which show that two of these parameters, G and G _c, take universal values in the . In this paper we present a detailed study of parameters measuring OPF for two mean-field models with and without time-reversal symmetry which exhibit different patterns of replica symmetry breaking below the transition: the Sherrington-Kirkpatrick model with and without a field and the Ising p-spin glass (p = 3). We give numerical results and analyze the consequences which replica equivalence imposes on these models in the infinite volume limit. We give evidence for the transition in each system and discuss the character of finite-size effects. Furthermore, a comparative study between this new family of parameters and the usual Binder cumulant analysis shows what kind of new information can be extracted from the finite T behavior of these quantities. The two main outcomes of this work are: 1) Parameters measuring OPF give better estimates than the Binder cumulant for T _c and even for very small systems they give evidence for the transition. 2) For systems with no time-reversal symmetry, parameters defined in terms of connected quantities are the proper ones to look at. Received 20 September 2000 and Received in final form 10 January 2001     

Theoretical and Experimental Investigation of Neon-Hydrogen Penning Recombination Laser

A theoretical and experimental study of Ne - H₂ Penning Recombination Laser operating on the Nel 585.3 nm line in the negative glow of a helical hollow cathode discharge is presented. A detailed kinetic model of the plasma-chemical reactions determining the inversion population on the Nel (2p₁-1s₂) transition is developed. The time dependent behaviour of the main plasma and laser parameters is numerically calculated. The behaviour of the plasma and laser parameters as a function of the discharge conditions is investigated and compared with the experimental results.     

Pressure effect on magnetic and insulator–metal transition of La0.67Pb0.33Mn0.9Co0.1O2.97 ceramic

In our paper, we study the effect of hydrostatic pressure on the magnetic and insulator–metal transitions of La_0.67Pb_0.33Co_0.1Mn_0.9O_2.97 manganite. The insulator–metal transition does not coincide with the magnetic transition in this ceramic. The compound undergoes the paramagnetic–ferromagnetic phase transition at T _C=260 K and the insulator–metal transition at T _p=184?K at ambient pressure. We have found that both characteristic temperatures T _C and T _p increase with increasing hydrostatic pressure with Δ T _C/Δ p=11.2 K/GPa and Δ T _p/Δ p=20.1 K/GPa, respectively.     

Computational study of collisional energy transfer and quasi-continuous population inversions in laser-pumped Ca vapor

A computational study of population inversion between several pairs of excited states viz 3d4p ³ F-4s3d ³ D, 4s5s ³ S-4s4p ³ P and 4s3d ³ D-4s4p ³ P in Ca vapor pumped on the 4s ^{2 1} S ₀-4s4p ³ P ₁ transition is presented. The main aim is to investigate the influence of various atomic processes in creating and sustaining the population inversion for long times after the excitation pulse. The delicate interplay between superelastic energy transfer to free electrons, energy pooling collisions and cascaded recombination is particulary examined. It is noted that quasi-continuous population inversion can be readily excited on the 4s3d ³ D-4s4p ³ P transitions; and under some conditions, also on the 4s5s ³ S-4s4p ³ P transitions. Furthermore, inversion on the 3d4p ³ F-4s3d ³ D transitions can also be excited for a considerable length of time. The results may be useful in designing and developing quasi-cw metal vapor lasers.     

Influence of quenched nonmagnetic impurities on phase transitions in a three-dimensional diluted Potts model with spin state number q = 4

The influence of quenched nonmagnetic impurities on phase transitions and critical phenomena in the 3D Potts model with the spin state number q = 4 is studied using the Monte Carlo method. Systems with the linear size L = 20–32 and spin concentrations p = 1.00, 0.90, 0.65 are considered. The fourth order Binder cumulant method is used to demonstrate that in the strongly diluted regime, a phase transition of the second kind is observed in this model for the spin concentration p = 0.65, and a phase transition of the first kind is observed for the pure (p = 1.00) and weakly diluted (p = 0.90) models. The theory of finite-dimensional scaling is used to calculate the static critical parameters of heat capacity α, susceptibility γ, magnetization β, and correlation radius ν.     

Extension of the sum rule for the transition rates between multiplets to the multiphoton case

The sum rule for the transition rates between the components of two multiplets, known for the one-photon transitions, is extended to the multiphoton transitions in hydrogen and hydrogen-like ions. As an example the transitions 3p–2p, 4p–3p and 4d–3d are considered. The numerical results are compared with previous calculations.     

Influence of nanoparticle size on the pH-dependent structure of adsorbed proteins studied with quantitative localized surface plasmon spectroscopy

We have studied p H-dependent conformational transitions of Bovine Serum Albumin adsorbed onto different sizes of gold nanospheres. For larger spheres (D > 10 nm) there is evidence for a path-dependent extended state near p H 4, over a very small p H range. For smaller nanospheres (5nm and 10nm) the evidence for such a transition is either much weaker or completely suppressed. We suggest that the absence of the transition on small spheres is due to the fact that the protein adsorbed on such small spheres has already lost at least some of its tertiary structure. The results have important implications for the functionality of proteins adsorbed onto nanospheres or surfaces with nm scale roughness.     

A visible helium plasma recombination laser

A small (active volume ≈ 6 cm³), fast Blümlein type, pulsed, transverse-discharge driven laser device exhibited afterglow lasing on the 3s³S-2p³P He transition at 706.5 nm in mixtures of helium and hydrogen or deuterium over a pressure range of 200 mbar. The main features of the laser pulses and relevant time-resolved fluorescence investigations are presented. Population inversion is attributed to the enhanced recombination pumping of upper level population and to hampered lower level population in the presence of H₂ (D₂).