分子磁体中的量子隧穿及宏观量子效应

文章介绍了分子磁体中的量子隧穿和宏观量子效应理论和实验研究的新进展．分子磁体既有宏观磁体特性也呈现纯量子行为，例如磁化矢量的量子隧穿．文章作者解释了如何通过量子隧穿实现宏观量子相干(即薛定谔猫态的相干叠加)和量子态位相干涉．对隧穿率计算的瞬子方法，特别是有限温度隧穿理论及其在分子磁体量子隧穿中的应用也做了简要的阐述．     

能级填充对量子阱光学性质的影响

在特殊设计的三势垒双势阱结构中，利用来自发射极的电子注入和电子向收集极的共振隧穿 逃逸调控量子阱不同子能级上的填充状态，发现激发态上的电子占据起抑制量子限制Stark 效应的作用.在极低偏压下，量子阱中少量过剩电子诱发了用简单带—带跃迁无法解释的光致发光光谱行为. 关键词： 共振隧穿 光致发光 量子限制Stark效应 过剩电子     

声子对隧穿量子点分子辐射场系统量子相位的影响

用全量子理论导出隧穿量子点分子-辐射场相互作用系统状态满足的微分方程组, 在相干态辐射场和量子点分子处于隧穿激发态及基态的初始条件下, 应用Pegg-Barnett相位理论计算和分析了辐射场的相位概率分布及相位涨落, 研究了声子-量子点分子作用对辐射场相位的影响, 并与Husimi相位分布做了比较. 结果表明, 温度显著影响光场相位概率分布的时间演化规律, 声子既可以抑制也可以增强辐射场相位扩散和涨落, 取决于量子点分子的初态. Husimi相位分布和Pegg-Barnett相位分布符合度相当高. 关键词： 量子点分子 声子 量子相位 Q函数')" href="#">Q函数     

抛物线性限制势二能级系统量子点量子比特的温度效应

应用Pekar变分方法,在抛物量子点中电子与体纵光学声子强耦合条件下,得出了电子的基态和第一激发态的本征能量及基态和第一激发态的本征波函数.以量子点中这样的二能级体系作为一个量子比特.当电子处于基态和第一激发态的叠加态时,计算出电子在时空中作周期性振荡的概率分布.并且得出了概率分布随温度及耦合强度的变化关系. 关键词： 量子点 量子比特 温度效应     

宏观共振隧穿的新进展及其在量子信息处理中的应用

从量子力学诞生以来,关于宏观物体的运动是否遵循量子力学的辩论就一直没有停止过.上世纪八十年代初期以来,一系列在约瑟夫森结和超导最子干涉器件(SQUID)中观测到的实验结果,包括相位和磁通的宏观量子隧穿,能级量子化,宏观共振隧穿,和在微波驱动下的相干动力学过程对认为宏观物体的运动在满足一定条件下同样遵循量子力学规律的观点提供了强有力的实验证据.在众多已观察到的宏观量子现象中,宏观共振隧穿结合了能级分立和隧穿这两个最具特征的量子现象.由于宏观共振隧穿的观测无需使用高频电磁波激发,这就避免了实验结果也可以用经典物理解释的可能,所以在一个系统中观测到宏观共振隧穿可以说是展示该系统的量子属性的最有力证据.本文讨论近年来从理论和实验两方面理解耗散和磁通噪声对类似SQUID的双势阱系统宏观共振隧穿率和谱线形状的影响.评述宏观共振隧穿谱的测量在探寻、理解、克服超导磁通量子比特中的退相干机制并最终实现规模化量子计算方面的应用.     

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.     

ZnCdSe量子阱/CdSe量子点耦合结构中的激子隧穿过程

用室温光致发光谱和飞秒脉冲抽运探测方法对不同垒宽的ZnCdSe量子阱/ZnSe/CdSe 量子点新型耦合结构中激子隧穿过程进行研究，观察到激子从量子阱到量子点的快速隧穿过 程.在ZnSe垒宽为10nm, 15nm, 20nm时，测得激子隧穿时间分别为1.8ps, 4.4ps, 39ps. 关键词： ZnCdSe量子阱 CdSe量子点 激子 隧穿     

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。     

ZnCdSe/ZnSe非对称双量子阱中的光学特性研究

用LP-MOCVD技术在GaAs衬底上外延生长了ZnCdSe/ZnSe非对称双量子阱(ADQW)结构。通过ps时间分辨光谱、吸收光谱、发射光谱等的研究得到了如下的结果:在弱激发下,观测到ADQW结构中的激子隧穿现象;在强激发下,在ADQW结构中发现了一个内建电场,它将影响激子隧穿;首次观测到由激子隧穿引起的在一定温度范围内宽阱的发光强度随温度上升而增加的现象;首次观测到该ADQW结构中来自宽阱的光泵受激发射。     

精确到二级近似下的量子隧穿概率

利用WKB法将量子隧穿计算到二级近似,给出了二级近似下粒子隧穿势垒的概率.     

PATH INTEGRAL CALCULATION OF QUANTUM TUNNELING FOR CUBIC POTENTIAL AT FINITE TEMPERATURE

The periodic instanton method is used to study the decay rates of metastable ground state and excited states of the cubic potential. The imaginary part of the energy is calculated through the standard procedure in the path-integral scheme. A formula of the decay rate valid for the entire region of energy is obtained. This formula provides a linkage between classical thermal activation at high temperatures and purely quantum tunneling at zero temperature. It is shown that in the low energy limit this more general result reduces exactly to the vacuum result. The temperature dependence of the decay rate agrees with earlier works in the literature.     

InAs自组装量子点GaAs肖特基二极管中的电流输运特性

设计了含有InAs自组装量子点(SAQDs)的新型金属半导体金属隧穿结构,研究了其直流输运特性,观察到了电流迟滞回路现象.这种回路现象是由于紧邻金属肖特基接触的量子点充电和放电引起的,也可以说是由外加电压控制的量子点的单电子过程引起的.分析了量子点总体的充放电特性,量子点中电子在高电场下隧穿出量子点的概率变化决定了量子点的放电过程,而充电过程是由流过量子点层的二极管正向电流决定.理论拟合结果显示充电过程主要由于量子点基态能级俘获电子照成的,激发态对量子点充放电过程只有微弱影响. 关键词： 迟滞现象 自组装量子点 单电子过程     

RECOMBINATION DYNAMICS IN ASYMMETRIC COUPLED QUANTUM WELL STRUCTURES

The time-resolved photoluminescence (PL) investigations of asymmetric coupled quantum well structures have been carried out in this paper. The experimental results show that for a wider interbarrier thickness the PL in the AlGaAs well will decay almost as a single exponential function, and for a thin interbarrier thickness the PL decay in the AlGaAs well is not a single exponential: the decay rate decreases gradually. These results indicate that when the tunneling becomes the main decay process in the AlGaAs well, the "effective mass filtering" phenomenon will be displayed in the PL decay process, which decreases the tunneling rate. At low temperature, the PL decay time increases with the increase of temperature. With further increase of temperature, the PL decay time decreases because of the increase of the tunneling rate and nonradiative recombination coefficient.     

Path Integral Method for Tunneling of Spin Systems and the Macroscopic Quantum Effect of Magnetization

The quantum tunneling effect in a ferromagnetic particle which can be evaluated only for the ground state previously is extended to excited states within the framework of instanton method. The tunneling between n-th degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration with which a formula of level-splitting valid for entire energy region is derived. In low energy region periodic instanton results coincide exactly with those derived through the LSZ reduction procedure. The tunneling effect increases at excited states. These results should be useful in further investigation of phase transition problem in ferromagnetic particles.     

一类弯曲量子线的量子束缚态

对电子在弯曲量子线中的弹道输运性质进行了理论研究.弯曲量子线由T型量子线和单曲量子线组成.该有限长的量子结构分别与两半无限长的量子通道相连,当施加一偏压时,量子通道分别可作为电子的发射极和收集极.计算结果表明,当入射电子的能量小于量子结构横向上的第一个本征模时,电导存在两个峰.进一步指出,这些峰来自于电子共振隧穿量子结构中的量子束缚态.并详尽地讨论了这些量子束缚态的性质. 关键词： 量子束缚态 共振隧穿 电导 量子线     

Current-oscillator correlation and Fano factor spectrum of quantum shuttle with finite bias voltage and temperature

A general master equation is derived to describe an electromechanical single-dot transistor in the Coulomb blockade regime. In the equation, Fermi distribution functions in the two leads are taken into account, which allows one to study the system as a function of bias voltage and temperature of the leads. Furthermore, we treat the coherent interaction mechanism between electron tunneling events and the dynamics of excited vibrational modes. Stationary solutions of the equation are numerically calculated. We show that current through the oscillating island at low temperature appears to have step-like characteristics as a function of the bias voltage and the steps depend on the mean phonon number of the oscillator. At higher temperatures the current steps would disappear and this event is accompanied by the emergence of thermal noise of the charge transfer. When the system is mainly in the ground state, the zero frequency Fano factor of current manifests sub-Poissonian noise and when the system is partially driven into its excited states it exhibits super-Poissonian noise. The difference in the current noise would almost be removed for the situation in which the dissipation rate of the oscillator is much larger than the bare tunneling rates of electrons.     

自组织生长InAs/GaAs量子点发光动力学研究

介绍了最新发展的粒子数混合超快光谱测量技术,以及采用该技术对自组织生长InAs/GaAs量子点发光动力学的研究结果.实验发现,自组织InAs/GaAs量子点结构的发光寿命大约为1ns,与InAs层厚度关系不大;激子寿命与温度有一定的关系,但没有明显的实验证据表明与量子点的δ态密度有关;用粒子数混合技术,实验上可直接观察到量子点中载流子在激发态能级的态填充过程. 关键词：     

耗散量子隧道系统中的局域—退局域转变

通过考虑声子基态变迭积分对隧道参量的重整化效应,本文提出一种新的方法来研究玻色型耗散量子隧道系统中的局域-退局域转变。研究表明,已有的理论结果主要建立在位移声子态近似的基础上,而本文的方法可以用来研究其它声子基态(如位移压缩声子态)对局域-退局域转变的影响。 关键词：     

QUANTUM DEFECT THEORY: RYDBERG STATES OF THE NO MOLECULES

The NO molecule.in Rydberg states consists of a closed shell molecular ion and an excited electron in a Rydberg molecular bital. It is a comparatively simple but non-trivial excited molecule. Here, we present our theoretical calculation of the quantum defects of these Rydberg states. The theoretical results are in good agreement with the experimental data.     

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).