粒子在 Hénon-Heiles势中的逃逸动力学模拟

利用庞加莱截面和相空间轨迹方法对粒子在Hénon-Heiles势中的逃逸动力学进行了模拟.粒子的动力学性质敏感地依赖于粒子的能量.数值计算表明当能量很小时,粒子的运动是规则的;随着能量的增加,粒子的运动开始出现混沌.当能量增加到鞍点能Es时,几乎所有的相空间轨迹都是混沌的.当粒子的能量E>Es,粒子可以越过势阱发生逃逸.对于给定的大于Es的能量, 我们画出了粒子的逃逸-时间曲线和逃逸轨迹.我们的研究对于研究混沌传输和逃逸动力学具有一定的参考价值.     

平行电磁场中里德堡氢原子的自相似结构研究

利用半经典方法研究了平行电磁场中里德堡氢原子的分形自相似现象. 通过研究平行电磁场中里德堡氢原子的逃逸时间和初始出射角间的关系, 发现了逃逸时间图的自相似结构, 并通过研究与图中冰柱对应的逃逸轨道, 得到了自相似结构和逃逸轨道之间的关系, 发现了该类自相似逃逸轨道满足的规律. 进一步研究了标度能量和标度磁场对体系动力学的影响, 表明标度能量和标度磁场均控制体系的分形自相似结构. 当标度能量或标度磁场比较小时, 没有自相似现象, 随着标度能量或标度磁场的增大, 自相似出现, 体系变复杂.     

二维Hénon-Heiles势及其变形势体系逃逸率与分形维数的研究

采用Chin和Chen的动力学算法追踪粒子在体系中的运动情况, 首次研究并对比了粒子在Hénon-Heiles体系与变形Hénon-Heiles六边形体系中的混沌逃逸规律, 在Hénon-Heiles体系中, 对于不同能量范围, 分形维数与逃逸率随能量而改变, 但在变形Hénon-Heiles六边形体系中, 仅在低能区分形维数与逃逸率随能量的改变而变化, 而高能区逃逸率和分形维数趋于稳定值. 并且得到普遍规律, 即不同混沌体系中粒子的混沌逃逸率和粒子逃逸的分形维数呈现较强的线性相关性. 因而分形维数可以作为工具研究混沌体系中粒子的逃逸规律, 在介观器件设计中可以通过研究混沌电子器件的分形维数来表征粒子在器件中的传输行为.     

A new approach to the determination of the Fano factor for semiconductor detectors

Values of the Fano factor are widely scattered in the literature, indicating the difficulties in its determination. We chose to analyze both the escape peaks and parent peaks of Ge detector spectra where the parent peak penetration depth and the escape peak escape depth are much larger than the size of the incomplete charge collection region. Hence, the escape peaks are expected to be free of low‐energy tailing as, although the actually deposited energy is low, it is deposited beyond the incomplete charge collection region. It was found that the product of electron–hole pair creation energy (ε) and the Fano factor (F) has an energy dependence at low energies, as is expected from electron transport theories. Although ε is expected to have its own energy dependence, if a reference value of 2.96 eV is assumed for ε, then the Fano factor values varied between 0.059 and 0.083. The escape peak is less advantageous for Si, hence a different method was used for Si(Li) detectors. Assuming the reference value of 3.8 eV for ε, the Fano factor was found to be 0.063 at 5.9 keV x‐ray energy. We consider the Fano factors reported here as upper limits, rather than the mean values. Copyright © 2004 John Wiley & Sons, Ltd.     

Deterministic escape of a dimer over an anharmonic potential barrier

In the present paper we consider the deterministic escape dynamics of a dimer from a metastable state over an anharmonic potential barrier. The underlying dynamics is conservative and noiseless and thus, the allocated energy has to suffice for barrier crossing. The two particles comprising the dimer are coupled through a spring. Their motion takes place in a two-dimensional plane. Each of the two constituents for itself is unable to escape, but as the outcome of strongly chaotic coupled dynamics the two particles exchange energy in such a way that eventually exit from the domain of attraction may be promoted. We calculate the corresponding critical dimer configuration as the transition state and its associated activation energy vital for barrier crossing. It is found that there exists a bounded region in the parameter space where a fast escape entailed by chaotic dynamics is observed. Interestingly, outside this region the system can show Fermi resonance which, however turns out to impede fast escape.     

Escape of photoelectrons and Compton‐scattered photons from an HPGe detector

The response function of a planar HPGe detector due to escape of photoelectrons and Compton‐scattered photons was studied for a point source with 59.5 keV energy. It was shown that both mechanisms, in addition to Ge x‐ray escape, leading to partial deposition of energy, could be observed in the same experiment. A Monte Carlo program was used to investigate these components of the response function. The results indicate that although the escape of scattered photons and Ge x‐rays are of the same magnitude, the escape of photoelectrons plays a more important role in the efficiency of a detector. Copyright © 2003 John Wiley & Sons, Ltd.     

An investigation of x‐ray escape for an HPGe detector

X‐ray escape peaks for a planar HPGe detector were investigated for a point source of photons with 59.5 keV energy. The effect of the solid angle for the incident photons on the escape probability of Ge K x‐rays was studied using collimators with varying apertures. A Monte Carlo program was also developed to estimate the probability of escape for various mechanisms and routes. The experimental and simulation results showed that the escape probability is essentially constant for a wide range of collimator radii, but at first starts to increase and then becomes constant when the side surface of the detector becomes accessible for escape with larger collimators. Copyright © 2003 John Wiley & Sons, Ltd.     

Abrupt Convergence and Escape Behavior for Birth and Death Chains

We link two phenomena concerning the asymptotical behavior of stochastic processes: (i) abrupt convergence or cut-off phenomenon, and (ii) the escape behavior usually associated to exit from metastability. The former is characterized by convergence at asymptotically deterministic times, while the convergence times for the latter are exponentially distributed. We compare and study both phenomena for discrete-time birth-and-death chains on ℤ with drift towards zero. In particular, this includes energy-driven evolutions with energy functions in the form of a single well. Under suitable drift hypotheses, we show that there is both an abrupt convergence towards zero and escape behavior in the other direction. Furthermore, as the evolutions are reversible, the law of the final escape trajectory coincides with the time reverse of the law of cut-off paths. Thus, for evolutions defined by one-dimensional energy wells with sufficiently steep walls, cut-off and escape behavior are related by time inversion.     

Simulation of methods for a rapid determination of the energy spectrum of bremsstrahlung from electron accelerators

Semiempirical computational methods for determining the energy spectrum of bremsstrahlung from pulsed electron accelerators are proposed. Input information for employing these methods includes the energy spectrum of electrons in a pulse, the effective angle of their incidence on the target, and the parameters of the converter target. It is shown that the methods in question can be used in a rapid determination of bremsstrahlung spectra for an arbitrary angle of the escape of bremsstrahlung, including its escape in the forward direction. Among other things, this provides a solution to the problem of dosimetry in radiation tests of electron systems.     

Phase escape of current-biased Josephson junctions

Switching current distributions of an Nb/Al--AlO_x/Nb Josephson junction are measured in a temperature range from 25~mK to 800~mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.     

表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响

研究表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响.计算梯度掺杂透射式GaN光电阴极的电子能量分布及逸出几率,结果显示梯度掺杂与均匀掺杂相比,可以获得更大的电子逸出几率;I势垒对电子逸出几率的影响显著,而Ⅱ势垒影响较小.利用GaN光电阴极多信息量测试系统,测试两种GaN阴极样品的光电流.实验结果表明,梯度掺杂GaN样品比均匀掺杂电子逸出几率更大;单独进行Cs激活形成的I势垒对电子逸出几率有显著影响,而Cs/O共同激活形成的Ⅱ势垒对其影响较小.     

Numerical experiments of the planar equal-mass three-body problem: the effects of rotation

The planar three-body problem with angular momentum is numerically and systematically studied as a generalization of the free-fall problem (i.e., the three-body problem with zero initial velocities). The initial conditions in the configuration space exhaust all possible forms of a triangle, whereas the initial conditions in the momentum space are chosen so that position vectors and momentum vectors are orthogonal. Numerical results are organized according to the value of virial ratio k defined as the ratio of the total kinetic energy to the total potential energy. Final motions are mapped in the initial value space. Several interesting features are found. Among others, binary collision curves seem to spiral into the Lagrange point, and for large k, binary collision curves connect the Lagrange point and the Euler point. The existence of a lunar periodic orbit and a periodic orbit of petal-type is suggested. The number of escape orbits as a function of the escape time is analyzed for different k. The behavior of this number for different time and k shows most remarkably the effects of rotation of triple systems. The number of escape orbits increases exponentially for k相似文献   

Stay-eat or run-away: Two alternative escape behaviors

Pursuit-and-evasion behavior in groups of animals is a phenomenon that can be easily observed in nature. Immediate flight upon detection is a common but not exclusive response for prey with both stay-eat behavior and run-away behavior occur during the predation process. It remains unclear why these two contrasting survival tactics appear, what the triggering conditions are and what internal mechanisms are at play. Here we investigate the effect of energy level on the behavior of prey during predation. We find that (1) the optimal escape speed of prey is context-dependent rather than the fastest always producing the best chance of survival. (2) The stay-eat or run-away decision depends mainly on the maximum speed and the energy dissipation ratio of prey to predator. (3) Stay-eat behavior is more effective when the prey have a higher escape speed and a higher energy dissipation rate, where aggregation can induce this stay-eat behavior. The reported findings are not only of relevance when considering survival tactics in nature, but such an understanding is useful in the design of swarm robotic systems where energy conservation and task optimization could be incorporated into any escape and hunting strategies.     

Fractal dynamics in the ionization of helium Rydberg atoms

We study the ionization of helium Rydberg atoms in an electric field above the classical ionization threshold within the semiclassical theory.By introducing a fractal approach to describe the chaotic dynamical behavior of the ionization,we identify the fractal self-similarity structure of the escape time versus the distribution of the initial launch angles of electrons,and find that the self-similarity region shifts toward larger initial launch angles with a decrease in the scaled energy.We connect the fractal structure of the escape time plot to the escape dynamics of ionized electrons.Of particular note is that the fractal dimensions are sensitively controlled by the scaled energy and magnetic field,and exhibit excellent agreement with the chaotic extent of the ionization systems for both helium and hydrogen Rydberg atoms.It is shown that,besides the electric and magnetic fields,core scattering is a primary factor in the fractal dynamics.     

Escape dynamics in collinear atomic-like three mass point systems

The present paper studies the escape mechanism in collinear three point mass systems with small-range-repulsive/large-range-attractive pairwise interaction. Specifically, we focus on the asymptotic behaviour for systems with non-negative total energy.On the zero energy level set there are two distinct asymptotic states, called 1+1+1escape configurations, where all the three separations infinitely increase as t→∞. We show that 1+1+1 escapes are improbable by proving that the set of initial conditions leading to such asymptotic configurations has zero Lebesgue measure. When the outer mass points are of the same kind we deduce the existence of a heteroclinic orbit connecting the 1+1+1 escape configurations. We further prove that this orbit is stable under parameter perturbation.In the positive energies’ case, we show that the set of initial conditions leading to 1+1+1 escape configurations has positive Lebesgue measure.     

Isotope dependence of widths of hole analog states

The isotope dependence of the widths of the hole analog states is studied for the Zr, Mo and Sn isotopes. In order to calculate the proton escape widths, we propose a microscopic model, in which the coupling of a neutron hole state to the isobaric analog state of the core is considered. It is found that the proton escape widths carry the observed isotope dependence of the widths of the hole analog states and that the isotope dependence of the proton escape widths is caused by two effects: the isotope dependence of the maximum available energy for a decaying proton, and that of the occupation probabilities of the excess neutron orbits in the parent state.     

Stadium型介观器件腔中粒子逃逸率的研究

本文研究了粒子在二维弱开口的Bunimovich Stadium型介观混沌器件中的逃逸规律.利用经典统计的方法,通过改变器件端口宽度、圆弧半径及器件腔长等参数,首次发现随器件各项参数变化的分形维数与粒子逃逸率趋势符合,并揭示了混沌体系的逃逸指数受器件形状的影响.统计并拟合了粒子逃逸率与粒子波数大小的关系,数值结果表明,粒子逃逸率与波数为二次函数关系,但逃逸率与能量大小不是严格的线性关系.进一步分析了在器件入口处粒子的衍射效应对粒子逃逸的影响,结果表明,衍射效应使粒子逃逸率增加,且粒子数的演化在时间较短时不再满足指数关系,长时间的演化再次满足指数衰减规律.     

InAs quantum dots on different Ga(In)As surrounding material investigated by photoreflectance and photoluminescence spectroscopy: electronic energy levels and carrier’s dynamic

In this article, comprehensive combination of photomodulated optical spectroscopy (PR) and temperature-dependent photoluminescence (PL) is carried out to investigate the electronic energy levels and carrier dynamics in nanometers’ size InAs quantum dots (QDs) in different surrounding material. Depending on the temperature range, the integrated PL intensity as a function of temperature, correlated to a rate equation model reveals two thermal escape channels for the InAs QDs in a pure GaAs matrix and three thermal escape channels for InAs/InGaAs dots-in-a-well structure. The extraction of the electronic energy levels by room temperature PR allow analyzing the impact of the surrounding material composition on the thermal activation energies and resulting PL quenching process.     

Role of fluctuations in nonlinear dynamics of a driven polariton system in semiconductor microcavities

The escape time from the lower energy state of the bistable nonlinear driven microcavity oscillator has been obtained analytically by means of the quasi-classical kinetic equation in the basis of quasi-energy states. The dependence of the escape time on the intensity of the external field is in rather good agreement with the results of numerical experiments. Moreover, the numerical dependencies of the escape time on the damping parameter reveal a smooth crossover from exponential to diffusive-like behavior. The text was submitted by the authors in English.     

Activated escape of periodically driven systems

We discuss activated escape from a metastable state of a system driven by a time-periodic force. We show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude. This dependence is described by the logarithmic susceptibility, which is analyzed theoretically and through analog and digital simulations. A closed-form explicit expression for the escape rate of an overdamped Brownian particle is presented and shown to be in quantitative agreement with the simulations. We also describe experiments on a Brownian particle optically trapped in a double-well potential. A suitable periodic modulation of the optical intensity breaks the spatio-temporal symmetry of an otherwise spatially symmetric system. This has allowed us to localize a particle in one of the symmetric wells. (c) 2001 American Institute of Physics.