具有突触特性忆阻模型的改进与模型经验学习特性机理

许多忆阻器都具有与生物神经突触功能相似的特性,这些特性包括记忆与遗忘特性、经验学习特性等.文献[17]根据记忆与遗忘特性建立了这类忆阻器的模型,文献[19,20]在对该模型的仿真研究中发现该模型也具有描述经验学习特性的能力.在关于这一模型已有研究的基础上,本文对该模型状态方程的特性与机理给出进一步的分析.分析中发现原模型的窗口函数的设计和使用存在问题,并且原模型建模时对于实验现象的解读不够准确.针对这些问题对原模型的状态方程进行了改进,完善了模型功能.对于该模型能够描述经验学习特性的机理,分别利用对于模型的状态方程的分析以及周期脉冲信号作用下的状态方程解析分析,对该机理给出定性和定量的讨论.利用机理分析所得的相关结论,设计了基于经验学习实验的模型状态方程中的参数和函数的估计方法,方便了该模型在这一类忆阻器的实验研究中的应用.     

A scaling theory of bifurcations in the symmetric weak-noise escape problem

We consider two-dimensional overdamped double-well systems perturbed by white noise. In the weak-noise limit the most probable fluctuational path leading from either point attractor to the separatrix (the most probable escape path, or MPEP) must terminate on the saddle between the two wells. However, as the parameters of a symmetric double-well system are varied, a unique MPEP may bifurcate into two equally likely MPEPs. At the bifurcation point in parameter space, the activation kinetics of the system become non-Arrhenius. We quantify the non-Arrhenius behavior of a system at the bifurcation point, by using the Maslov-WKB method to construct an approximation to the quasistationary probability distribution of the system that is valid in a boundary layer near the separatrix. The approximation is a formal asymptotic solution of the Smoluchowski equation. Our construction relies on a new scaling theory, which yields critical exponents describing weak-noise behavior at the bifurcation point, near the saddle.     

INFLUENCE OF THE TEMPERATURE-DEPENDENCE OF RECOMBINATION LIFE-TIME OF FREE CARRIERS IN AMORPHOUS SEMICONDUCTORS ON THERMOSTIMULATED PHOTOCURRENT SPECTRA

In previous theoretical analysis of ther-mostimulated conductivity (TSC) in amorphous semiconductors, for the sake of simplicity, we assumed that the recombination life-time τ of free carriers is a constant independent of temperature. In fact, this assumption is incomplete. The influence of the temperature-dependence of T on thermostimulated conductivity spectra is investigated in the present work. Under the consideration of T-dependence of τ, all the characteristic of TSC spectra indicated by the previous theoretical analysis still remain valid qualitatively, but the quantitative results are considerably changed. The effect of the relaxation parameters describing the behavior of τ(T) on the CSC spectra is also discussed.     

Colored Noises-Induced Regime Shifts in a Vegetation Ecological System

In this paper, we investigate the stationary probability distribution and mean first passage time in a vegetation ecological system, which is driven by cross-correlation between intrinsic and extrinsic colored noises as well as the nonzero cross-correlation in between. The impacts of the self-correlation time τ₁ and τ₂, the cross-correlation time τ₃ and intensity k on the stationary probability distribution and mean first passage time are discussed, respectively. Our main results show that: (#em/em#) the self-correlation time τ₁ can induce regime shifts from the desert state to the sustainable vegetation state, while the self-correlation time τ₂, cross-correlation time τ₃ and intensity k can induce regime shifts from the sustainable vegetation state to the desert state; and (ii) the self-correlation time τ₁ or τ₂ can enhance the stability of the sustainable vegetation biomass, while the cross-correlation time τ₃ or strength k weakens the stability of the sustainable vegetation biomass.     

Optical study of charge dynamics in CaCo_2As_2

We present an infrared spectroscopy study of charge dynamics in CaCo_2As_2 single crystal. In this material, the optical conductivity can be described by two Drude components with different scattering rates(1/τ): a broad incoherent background and a narrow Drude component. By monitoring the temperature dependence, we find that only the narrow Drude component is temperature-dependent and determines the transport properties. Especially a Fermi liquid behavior of carriers is revealed by the T~2 behavior in the dc resistivity ρ_n and scattering rate 1/τ_n, indicating a coherent nature of quasiparticles in the narrow Drude subsystem.     

Effects of correlation time between noises on the noise enhanced stability phenomenon in an asymmetric bistable system

The effects of the correlation time τ between noises on the noise-enhanced stability (NES) phenomenon in an asymmetric bistable system driven by cross-correlated noise are investigated. The expressions for the average escape time from the left metastable state T_L and from the right metastable state T_Rare derived. The results indicate that: i) The NES effect is suppressed as the correlation time τ increases for two metastable states; ii) The increase in τ speeds up the escape process from the right state for positively correlated noise, whereas its role is reverses for negatively correlated; iii) In the escape process from the left state, the role of τ is opposite to that in escape from the right state.     

激光光源线宽对外差探测性能的影响

本文根据统计理论分析了激光线宽对外差探测结果的影响, 并讨论了激光线宽对杨氏干涉条纹对比度的影响. 本文基于解析结果做了数值仿真, 所得结果表明激光线宽增加时, 外差探测方式仍可以检测到中频信号, 但在线宽较大时, 受噪声影响无法准确地提取到中频频率. 为验证理论分析结果, 使用线宽为1 MHz的激光光源进行了8.1 km外场实验, 实验结果与数值分析结论一致, 即不会因为线宽增加而无法检测到中频信号. 文中所得结论对于外差探测光源的选择有重要指导意义, 因此根据测量目标的特性和测量要求, 可按照文中结论评估光源的线宽指标.     

The Spectral Density of Nonlinear O-U Noise and the Influence of Nonlinear Fluctuation on the Transition of Bistable System

A stochastic differential equation driven by N-order nonlinear fluctuation is investigated by defining a novel stochastic process Г(t). The spectral densities of Г(t) for N = 1,2,3 and 4 are obtained. We compare the results of the linear fluctuation case with the nonlinear fluctuation case and find that the extremum of the spectral density for linear case is directly proportional to the noise intensity D and independent of the correlation time τ, the extremum of the spectral density for nonlinear case is determined by both D and τ. An approximate Fockker-Planck equation of the N-order nonlinear fluctuation is derived by Hanggi-like ansatz. The infl uences of nonlinear fluct uation on the transition of bistable system are studied. It is found that the area of unimodal region and bimodal region of the stationary probability is independent of D and τ for linear case, however, the area of two regions is related to D and τ for nonlinear case.     

垂直气液两相流混沌吸引子单元面积分析

为了充分反映吸引子结构随时间延迟的变化规律,在现有吸引子形态描述方法基础上定义了吸引子单元面积,通过仿真发现,吸引子单元面积随时间延迟变化曲线第一个波峰的高度和时间延迟主要由信号中大幅值波动的数量、频率决定,利用此规律对实验采集到的气液两相流电导波动信号进行分析,发现在固定液相流量条件下,改变气相流量会导致泡状流、段塞流和混状流中大幅值波动幅度的改变,但相同流型信号中大幅值波动的频率比较接近.将吸引子单元面积随时间延迟变化曲线第一个波峰的时间延迟和落差比作为特征量,可以实现泡状流、段塞流、混状流的流型分类.     

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研究了在托卡马克装置中中性束注入时快离子自举电流的产生。利用快离子分布函数在两小增量δ=ρp/a (ρp是极向拉莫尔半径,a是小半径)和δ*=τB/τs(τB是俘获粒子反弹周期,τs是慢化时间)下进行展开的方法求解漂移动力学方程;给出了快离子自举电流的一般表达式。计算了圆截面大纵横比托卡马克中快离子自举电流密度分布和总的快离子自举电流的大小,研究表明：在中性束垂直注入时快离子自举电流约占总电流10%;自举电流的大小既敏感地依赖于中性束注入的角度——平行注入时较小、接近垂直时迅速增大,同时也较强的依赖于快离子的产生速度与临界速度的比值,即vb/vc,而vb2∝E(束能量);自举电流的大小随注入束能量的增加而迅速增大。     

Transport Induced by Dichotomic Temperature Fluctuations

The transport process of a Brownian particle in a ratchet-like potential subjected to two exchanging Gaussian white noises is studied. The change of the temperatures is described by a stochastic dichotornic process. By making use of the perturbation method the probability current up to second order in small correlation time τ is obtained, and time-averaged current is evaluated numerically for a whole region of the parameters. For asymptotically fst temperature cycles, the present model yields larger current in comparison with the periodical temperature cycles. Depending on the temperature of cold heat reservoir, the current changes sign as a function of τ.     

Testing discrete symmetries at a super τ -charm factory

Tests of discrete symmetry violation have played an important role in understanding the structure of weak interactions in the Standard Model of particle physics. Historically, these measurements have been extensively performed in experiments with large samples of K and B mesons. A high luminosity τ-charm facility presents physicists with the opportunity to comprehensively explore discrete symmetry violation and test the Standard Model using τ leptons, charm mesons, and charmed baryons. This paper discusses several possible measurements for a future τ-charm factory.     

表面活性剂CTAB水溶液中的T2弛豫分散研究

用CPMG脉冲序列测定了表面活性剂十六烷基三甲基溴化铵（CTAB）分子中的氮甲基(N-CH₃)质子的横向弛豫时间（T₂表观）,并发现测得的T₂表观\}与序列中的重聚脉冲间隔时间的一半τ _cp有关,说明存在横向弛豫分散现象. 当在τ_cp≤1 ms时,T₂表观与τ²_cp}呈线性关系;而当τ_cp≥4.6 ms时,T₂表观变得与τ_cp无关. 利用Luz-Meiboom两体化学交换模型计算了不同浓度的CTAB溶液中的N-CH₃质子的本征横向弛豫时间（T₂本征）和化学交换速率k_ex,发现k_ex与T₂本征和自扩散系数D一样,在临界胶束浓度（CMC）附近发生突变. 这个突变反映了CTAB分子在从单体到胶束的转变过程中其动力学特性发生了改变.      

Modeling of low-pressure microwave discharges in Ar, He, andO2: similarity laws for the maintenance field and mean powertransfer

A kinetic study of low-pressure microwave discharges in Ar, He, and O₂ is carried out using electron-transport parameters and rate coefficients derived from solutions to the Boltzmann equation, together with the continuity and transport equations for the charged particles, taking into account stepwise-ionization processes. The Boltzmann equation is solved over a wide range of the applied frequency, ω/2π, but assuming that the angular frequency ω>τ _e^-1, with τ_e, denoting the characteristic time for electron-energy relaxation by collisions. The formulation provides discharge characteristics for the maintenance field and for mean absorbed power per electron in the three gases, which are shown to agree satisfactorily with experimental data obtained from surface-wave discharges. It is shown that such an agreement would not always be obtained without consideration of the role played by stepwise-ionization processes in sustaining the discharge     

Kernel polynomial representation for imaginary-time Green's functions in continuous-time quantum Monte Carlo impurity solver

Inspired by the recently proposed Legendre orthogonal polynomial representation for imaginary-time Green s functions G(τ),we develop an alternate and superior representation for G(τ) and implement it in the hybridization expansion continuous-time quantum Monte Carlo impurity solver.This representation is based on the kernel polynomial method,which introduces some integral kernel functions to filter the numerical fluctuations caused by the explicit truncations of polynomial expansion series and can improve the computational precision significantly.As an illustration of the new representation,we re-examine the imaginary-time Green's functions of the single-band Hubbard model in the framework of dynamical mean-field theory.The calculated results suggest that with carefully chosen integral kernel functions,whether the system is metallic or insulating,the Gibbs oscillations found in the previous Legendre orthogonal polynomial representation have been vastly suppressed and remarkable corrections to the measured Green's functions have been obtained.     

General scaling of pulse shortening in explosive-emission-drivenmicrowave sources

Microwave generation in devices that depend on synchronization between an electron beam and a resonant cavity or slow wave structure can be disrupted by changes in either. Explosive-emission-driven microwave sources use plasma as the electron source in the diode. This plasma is conductive enough to act as the boundary for both the applied diode voltage and the microwave electric field. The motion of this plasma can effectively change the dimensions of either the electron beam diode or the cavity and will thereby cause resonance destruction. This shortens the microwave pulse length τ_μ. A general model of the process predicts that, for a Child-Langmuir diode, microwave power falls as P∝τ_μ^-5/3 and that pulse energy falls as E∝τ_μ^-2/3. Therefore, energy efficiency declines as the pulse length is extended. We compare with data from magnetrons, MILO's and BWO's, and find that over some regions of operation the pulse length and energy from these explosive-emission-driven microwave sources agree with the plasma motion model scaling. At these applied drive voltages and output powers the microwave pulse length can be increased by finding cathode materials that generate slower plasmas     

Two-Mode Wigner Operator in 〈ξ| Representation and 〈τ| Representation

We derive the two-mode Wigner operator in the 〈ξ| representation and 〈τ| representation, where |ξ〉 is common eigenvector of the mass-weighted relative coordinate and the mass-combinatorial momentum.and |τ〉 common eigenvector of the mass-weighted relative coordinate and the mass-combinatorial momentum. As an application,we calculate the Wigner function of some two-mode state.     

Lepton Flavor Violating Decays τ→μM in the 331 Model

Considering the experimental constraints on the free parameters of the 331 model with a leptonic sector consistent of five triplets, we investigate the lepton flavor violation (LFV) tau decays τ→μM with M=P and V, where P and V denote a pseudoscalar meson (π,η or η') and a vector meson (ρ⁰,ω or φ), respectively. We find that the contributions of the 331 model to the LFV decays τ→μM mainly come from the new neutral gauge boson Z'. The 331 model considered in this paper can not make the values of the branching ratio Br(τ→μM) approach the corresponding experimental upper limits.     

Theory of bio-energy transport in protein molecules and its experimental evidences as well as applications ( I )

A new theory of bio-energy transport along protein molecules, where energy is released by the hydrolysis of adenosine triphosphate (ATP), has recently been proposed for some physical and biological reasons. In this theory, Davydov’s Hamiltonian and wave function of the systems are simultaneously improved and extended. A new interaction has been added into the original Hamiltonian. The original wave function of the excitation state of single particles has been replaced by a new wave function of the two-quanta quasi-coherent state. In such a case, bio-energy is carried and transported by the new soliton along protein molecular chains. The soliton is formed through the self-trapping of two excitons interacting with amino acid residues. The exciton is generated by the vibration of amide-I (C=O stretching) arising from the energy of the hydrolysis of ATP. The properties of the soliton are extensively studied by analytical methods and its lifetime for a wide range of parameter values relevant to protein molecules is calculated using the nonlinear quantum perturbation theory. The lifetime of the new soliton at the biological temperature of 300 K is large enough and belongs to the order of 10^-10 s or τ/τ₀ ≥ 700. The different properties of the new soliton are further studied. The results show that the new soliton in the new model is a better carrier of bio-energy transport and it can play an important role in biological processes. This model is a candidate of the bio-energy transport mechanism in protein molecules.     

Evaluation of the frequency instability limited by Dick effect in the microwave ~(199)Hg~+ trapped-ion clock

In the microwave ~(199)Hg~+ trapped-ion clock, the frequency instability degradation caused by the Dick effect is unavoidable because of the periodical interrogating field. In this paper, the general expression of the sensitivity function g(t)to the frequency fluctuation of the interrogating field with Nπ-pulse(N is odd) is derived. According to the measured phase noise of the 40.5-GHz microwave synthesizer, the Dick-effect limited Allan deviation of our ~(199)Hg~+ trapped-ion clock is worked out. The results indicate that the limited Allan deviations are about 1.75 × 10~(-13)τ~(1/2) and 3.03 × 10~(-13)τ~(1/2) respectively in the linear ion trap and in the two-segment extended linear ion trap under our present experimental parameters.