非线性啁啾频率对势阱产生正负电子对的增强

用计算量子场论方法研究了非线性啁啾频率对势阱中正负电子对产生的增强效应。研究了由静态势阱和动态势阱组成的组合势阱中产生的正负电子对的密度、产额和能谱等性质随着啁啾参数的变化,分析了组合势阱的频谱和瞬时束缚态。发现非线性啁啾效应对低频区域比较敏感,与固定频率情况相比可以使粒子数增加2～3倍。与组合势阱相比,非线性啁啾效应对单个振荡势阱更敏感。在低频下单个振荡的势阱中正负电子对产额可提高多个数量级。这是因为在低频下单个振荡的势阱中,主要通过量子隧穿过程产生的正负电子对数目非常低。非线性啁啾效应增加了高频场成分,提高了多光子过程和动力学辅助机制。由于高频抑制作用,所以非线性啁啾效应对高频区域粒子的增量不大,甚至会抑制正负电子对的产生。     

费米-费米散射长度对费米超流气体在幺正极限区域的隧穿现象影响

运用双势阱模型通过调节费米-费米散射长度研究了超流费米气体在幺正极限区域的隧穿现象. 研究发现费米-费米散射长度对量子隧穿效应有显著的影响. 在确定的区域, 可以得到在双势阱中完全的量子隧穿现象, 而在另一些区域, 这种隧穿就完全消失. 由于在实验室中超流费米气体的散射长度是可以调节的, 所以在实验上可以实现通过调节散射长度来控制超流费米气体的宏观隧穿现象.     

量子相空间中对称受驱双势阱系统的量子隧穿动力过程

本文以纠缠轨线分子动力学方法研究对称受驱双势阱系统的量子隧穿动力学过程.驱动力的幅度和频率改变将对量子隧穿动力学过程产生巨大的影响,这为人们自主控制这一重要的过程提供理论基础.当体系的经典动力学呈现混沌状态时,它的量子动力学过程将发生显著的变化.在强驱动力作用下,双势阱系统的量子共振频率隧穿和非共振频率隧穿因为混沌行为的出现明显增强.通过对比相空间中具有相同初始态的纠缠轨线和经典轨线演化,我们给出量子隧穿过程清晰的物理图像.最后,我们讨论量子隧穿动力学过程中体系不确定度的演化和反映波包动力学过程的自关联函数演化.     

玻色-爱因斯坦凝聚体系中的混沌隧穿行为

研究了双势阱玻色-爱因斯坦凝聚(BEC)系统在外加周期调制下的混沌相变过程,着重讨论了混沌现象对BEC系统隧穿的影响.当外加调制频率与系统固有频率达到共振时,相平面会出现不稳定现象,即混沌现象.在量子情况下,研究了系统的Husimi函数随时间的演化.研究发现：当混沌现象出现时,系统中粒子间相互作用增大,使得混沌区域扩大,进而引起混沌辅助隧穿程度的变化. 关键词： 玻色-爱因斯坦凝聚 混沌 隧穿     

双势垒抛物势阱磁性隧道结隧穿磁阻及自旋输运性质的研究

采用相干量子输运理论和传递矩阵的方法,在抛物势阱磁性隧道结(F/PW/F)的铁磁和半导体势阱间插入另一种半导体作为势垒,构造具有双势垒的抛物势阱磁性隧道结作为研究对象,研究了抛物势阱宽度、自旋轨道耦合效应、角度效应及插入势垒厚度对隧穿磁阻及自旋输运性质的影响计算结果表明,通过适当调节Rashba自旋轨道藕合强度和插入势垒的厚度,可以实现隧穿磁阻(TMR)的调制,能获得较大的TMR值,这些特点有助于促进新型磁性隧道结的开发和应用.     

玻色-费米超流混合体系中的相互作用调制隧穿动力学

研究了玻色-费米超流混合体系中的相互作用调制隧穿动力学特性,其中玻色子位于对称双势阱中,费米子位于对称双势阱中心的简谐势阱中.采用双模近似方法得到描述双势阱玻色-爱因斯坦凝聚的动力学特性方程组,并将其与简谐势阱中分子玻色-爱因斯坦凝聚的Gross-Pitaevskii方程进行耦合.通过对不同参数下玻色-费米混合体系中的隧穿现象进行数值研究,发现简谐势阱中费米子与双势阱中玻色子的相互作用使双势阱玻色-爱因斯坦凝聚的隧穿动力学特性更加丰富.不但驱使双势阱中玻色-爱因斯坦凝聚从类约瑟夫森振荡转变为宏观量子自囚禁,而且宏观量子自囚禁表现为三种不同的形式:相位与时间呈负相关并随时间单调减小的自囚禁、相位随时间演化有界的自囚禁以及相位与时间呈正相关并随时间单调增大的自囚禁.     

对称抛物势阱磁性隧道结中的自旋输运及磁电阻效应

研究了两端具有铁磁接触的对称抛物势阱磁性隧道结(F/SPW/F)中自旋相关的隧穿概率和隧穿磁电阻,讨论了量子尺寸效应和Rashba 自旋轨道耦合作用对自旋极化输运特性的影响.研究结果表明:隧穿概率和隧穿磁电阻随抛物势阱宽度的增加发生周期性的振荡.抛物势阱深度的增加减小了隧穿概率和隧穿磁电阻的振荡频率.Rashba 自旋轨道耦合强度的增加加大了隧穿概率和隧穿磁电阻的振荡频率.隧穿概率和隧穿磁电阻的振幅和峰谷比强烈依赖于两铁磁电极中磁化方向的夹角. 关键词： 磁性隧道结 Rashba 自旋轨道耦合 隧穿概率 隧穿磁电阻     

玻色-爱因斯坦凝聚体在光势阱中的量子隧穿

微观粒子因具有波动性而能以一定的概率穿过比它动能更高的势垒，这一现象称为隧道效应，玻色-爱因斯坦凝聚体在不同的囚禁势阱中表现出不同的隧穿特性，文章作者用周期瞬子方法首次研究了光势阱中玻色-爱因斯坦凝聚体的量子隧穿，研究表明，处在光势阱中的玻色-爱因斯坦凝聚体不仅表现出Landau-Zenner隧穿，而且出现新的Wannier-Stark隧穿，Wannier-Stark隧穿系数大约是Landu-Zener隧穿系数的1000倍，并得到了隧穿率随温度的变化规律，包括经典热激活、热助隧穿和量子隧穿，理论的计算结果与Yale大学和意大利INFM研究组关于Landau-Zener隧穿的实验结果相符合，同时给出了如何在实验上发现Wannier-Stark隧穿的参数区间。     

有效质量法调控原子玻色-爱因斯坦凝聚体的双阱动力学

操控原子玻色-爱因斯坦凝聚体在双势阱中的动力学通常是通过改变势阱深度来实现,本文提出了一种基于调节原子有效质量的控制方案,可以在不改变双阱势的前提下操控凝聚体的双阱动力学.利用双模近似,本文解析地导出了超冷原子在双阱势中的隧穿强度和相互作用强度对有效质量的依赖关系,并基于平均场近似数值模拟了在有效质量调节下的凝聚体动力学演化,展示了隧穿振荡和自束缚等典型的双阱动力学行为.此外,本文的研究还发现,借助负有效质量效应,这一方案甚至可以等效地实现对负散射长度原子凝聚体双阱动力学行为的操控.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

Nonlinear Propagation of Positron-Acoustic Periodic Travelling Waves in a Magnetoplasma with Superthermal Electrons and Positrons

The nonlinear propagation of positron acoustic periodic(PAP) travelling waves in a magnetoplasma composed of dynamic cold positrons, superthermal kappa distributed hot positrons and electrons, and stationary positive ions is examined. The reductive perturbation technique is employed to derive a nonlinear Zakharov-Kuznetsov equation that governs the essential features of nonlinear PAP travelling waves. Moreover, the bifurcation theory is used to investigate the propagation of nonlinear PAP periodic travelling wave solutions. It is found that kappa distributed hot positrons and electrons provide only the possibility of existence of nonlinear compressive PAP travelling waves. It is observed that the superthermality of hot positrons, the concentrations of superthermal electrons and positrons, the positron cyclotron frequency, the direction cosines of wave vector k along the z-axis,and the concentration of ions play pivotal roles in the nonlinear propagation of PAP travelling waves. The present investigation may be used to understand the formation of PAP structures in the space and laboratory plasmas with superthermal hot positrons and electrons.     

Magnetic confinement of massless Dirac fermions in graphene

Because of Klein tunneling, electrostatic potentials are unable to confine Dirac electrons. We show that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields. This allows one to design mesoscopic structures in graphene by magnetic barriers, e.g., quantum dots or quantum point contacts.     

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: I. Electron-positron pair production

The production of electron-positron pairs in a vacuum neutron star magnetosphere is investigated for both low (compared to the Schwinger one) and high magnetic fields. The case of a strong longitudinal electric field where the produced electrons and positrons acquire a stationary Lorentz factor in a short time is considered. The source of electron-positron pairs has been calculated with allowance made for the pair production by curvature and synchrotron photons. Synchrotron photons are shown to make a major contribution to the total pair production rate in a weak magnetic field. At the same time, the contribution from bremsstrahlung photons may be neglected. The existence of a time delay due to the finiteness of the electron and positron acceleration time leads to a great reduction in the electron-positron plasma generation rate compared to the case of a zero time delay. The effective local source of electron-positron pairs has been constructed. It can be used in the hydrodynamic equations that describe the development of a cascade after the absorption of a photon from the cosmic gamma-ray background in a neutron star magnetosphere.     

On the positron in a metal

The self-consistent calculations of the spatial distributions of electrons and potentials in vacancies of metals with a localized positron, the spatial distributions of positrons localized in surface states, and the binding energies of positrons and their lifetimes have been performed in terms of the Kohn-Sham method and the stabilized jellium model. The presence of a localized positron in a vacancy leads to the effect that the vacancy is weakly distinguishable for electron waves: the positron weakens the potential field in the vicinity of the vacancy and leads to a phase shift of the scattering electron wave functions. The calculation of the phase shifts of the wave functions for quasi-free positrons scattered by unperturbed vacancies and the representation of a system of vacancies as a “superlattice” in a metal have made it possible to find the shift of the positron work function and the vacancy contribution to the positron effective mass.     

Resonant tunneling through double-barrier structures on graphene

Quantum resonant tunneling behaviors of double-barrier structures on graphene are investigated under the tightbinding approximation. The Klein tunneling and resonant tunneling are demonstrated for the quasiparticles with energy close to the Dirac points. The Klein tunneling vanishes by increasing the height of the potential barriers to more than 300 meV. The Dirac transport properties continuously change to the Schro¨dinger ones. It is found that the peaks of resonant tunneling approximate to the eigen-levels of graphene nanoribbons under appropriate boundary conditions. A comparison between the zigzag- and armchair-edge barriers is given.     

Theoretical analysis of planar and non‐planar electron ‐ acoustic shock waves in electron‐positron‐ion plasma

The propagation properties of planar and non‐planar electron acoustic shock waves composed of stationary ions, cold electrons, and q‐non‐extensive hot electrons and positrons are studied in unmagnetized electron‐positron‐ion plasma. In this model, the Korteweg‐de Vries Burgers equation is obtained in the planar and non‐planar coordinates. We have investigated the combined action of the dissipation, non‐extensivity, density ratio of hot to cold electrons, concentration of positrons, and temperature difference of cold electrons, hot electrons, and positrons. It was found that the amplitude of shock wave in e‐p‐i plasma increases when the positron concentration and temperature increase. The same effect is observed in the case of kinematic viscosity η. Furthermore, it is noticed that spherical wave moves faster in comparison to the shock waves in cylindrical geometry. This difference arises due to the presence of the geometry term m/2τ. It should be noted that the contribution of the geometry factor comes through the continuity equation. Results of our work may be helpful to illustrate the different properties of shock wave features in different astrophysical and space environments like supernova, polar regions, and in the vicinity of black holes.     

Production of intense positron beams at the VEPP-5 injection complex

A source of positrons allowing 5 × 10⁸ positrons accelerated to the energy of 70 MeV to be produced per pulse has been developed. The process of electron-positron pair production in an electromagnetic shower is used for production of positrons. The electromagnetic shower is generated in a tantalum target by a beam of 2 × 10¹⁰ electrons with energy 270 MeV. A high efficiency of positron collection (positron yield Y ≈ 0.1 GeV^?1) is ensured by a unique design of the matching device.     

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

超短超强激光产生正电子的蒙特卡罗模拟

通过理论分析,建立了超短超强激光与固体靶作用产生正电子的蒙特卡罗模拟模型及Geant4模拟程序。模拟研究了靶材料、靶厚度及超热电子温度等对正电子产额的影响,结果表明:对铝、铜、锡、钽、金、铅6种靶材料,金靶的正电子产额最高,是优秀的正电子产生靶;不同超热电子温度下存在不同的最佳靶厚度,在最佳靶厚度以下,正电子产额随靶厚度增长而增大,靶厚度取3 mm较为合适;超热电子温度越高,正电子产额也越高,提高激光强度是增加正电子产额的有效途径。模拟研究给出了正电子角分布及其能谱,结果显示,正电子发射明显前倾,从大于90方向范围发射的正电子数量极少,且超热电子温度越高前倾特点越明显,能量呈类麦克斯韦分布,靶背法线方向出射的正电子的温度随超热电子温度升高而升高。