在强交变电场驱动下线形三量子点分子中激子的动态局域化行为

采用三点Hubbard模型和Floquet定理，研究了在强交变电场驱动下线形三量子点分子中激子的动力学行为.数值计算表明，尽管驱动电场倾向使电子和空穴在空间上分离，但在合适的 外电场驱动下，初始局域在一个量子点中的电子和空穴依旧保持其初始局域状态.应用微扰 理论求解Floquet方程，从而得到准能的解析解.表明在准能级发生交叉点，激子可呈现出动 态局域化. 关键词： Floquet态 准能 局域态 动态局域化     

交变电场驱动下三量子点中双电子的动力学

采用三点哈巴德模型及弗洛盖定理．研究了交变电场驱动下线形三量子点分子中双电子的动力学行为。由于系统哈密顿量中不包含自旋反转项,所以系统单态和三重态子空间是完全解耦的,可以分开进行讨论。研究表明,自旋三重态九维子空间还可以进一步分解成三个不相耦合的子空间,在每一个子空间中,动力学行为与交变电场驱动的双量子点中双电子的动力学行为相似。对自旋单态6维子空间,数值计算还表明,在合适的外加交变电场驱动下,电子在量子点之间的隧穿被抑制．初始局域在一个量子点中的两个电子能够在一定时间内保持其局域状态。     

光子晶体中二能级原子的自发辐射

研究光子晶体中二能级原子的自发辐射。由于光的局域化以及缀饰态之间的量子相互作用 ,由局域场、传输场、弥散场组成的原子的辐射场发生变化 :随着激发态能级由禁带移往通带 ,发现局域场中的能量通过弥散场向传输场转移。当局域场和传输场共存时 ,激发态能级上的原子布居数的演化具有准振荡行为 ;当局域场和传输场不共存时 ,原子布居数的演化不具有准振荡行为。另外 ,激发态能级上的稳态原子布居数也发生改变。这些性质不仅依赖于原子的激发态能级ω1与光子频率能带边缘ωc 的相对位置δ,同时也依赖于光子晶体中带边缘的光子态密度 ρ(ωk) (或带边光滑参数ε)。     

随机激光器中准态腔的阈值与其局域化程度的关系

基于光波在有限随机介质中的局域化理论，利用有限时域差分法数值求解Maxwell方程 组，研究了随机介质中的激光现象，分析了准态模的放大与其 空间局域性的关系. 通过研究二维非增益随机介质中光波的局域化，确定了准态模的空间分 布和频谱特征. 通过引入增益，研究了准态模的放大过程和阈值特性. 结果表明空间局域化 强的准态模在增益介质中被优先放大，且有较低的阈值. 关键词： 随机激光器 准态模 随机介质中的光学特性     

点间耦合强度对三耦合量子点系统微分电导的影响

本文利用非平衡格林函数运动方程方法,研究了与两个电极耦合在一起的三耦合量子点系统的微分电导及量子干涉的AB振荡问题.通过理论计算发现,由于量子点上的局域态密度的不同从而导致系统电导或隧穿性质的不同,而且量子点间耦合强度、量子点能级等都会对输运性质产生影响. 关键词： 量子点 非平衡格林函数 运动方程 局域态密度     

DNA分子链电子结构特性研究

在单电子紧束缚近似下，建立了一维无序二元DNA分子链模型，计算了链长为2×10⁴个碱基对的DNA分子链的电子态密度、局域化特性，并探讨了碱基对的不同组分、格点能量无序度对电子局域态的影响.结果表明：由于DNA分子链中格点能量无序及碱基对的不同组分的存在，其电子波函数呈现出局域化的特性，而局域长度作为衡量电子局域化程度的一个尺度，受碱基对的组分及格点能量无序度的影响. 关键词： DNA分子链 电子结构 电子局域态 局域长度     

耦合锗量子点中空穴态对称特性研究

分别采用单带重空穴近似和六带Kronig-Penney模型, 对垂直耦合锗量子点在不同耦合距离下的空穴态特性进行了计算, 并探讨了自旋-轨道的相互作用对空穴态对称性的影响. 计算结果表明: 多带耦合的框架下, 随着量子点垂直间距的增大, 空穴基态从成键态转变为反键态, 而且价带基态能级和第一激发态能级发生反交叉现象, 这与单带模型下得到的相应结果存在较大差异. 通过分析六带模型计算得到的成、反键态波函数, 轻、重空穴态和自旋-轨道分裂态对特征空穴态波函数的贡献比例随着量子点垂直间距的增大发生了转变, 并最终导致量子点空穴基态波函数由成键态转变为反键态. 关键词： 耦合量子点 空穴态 成健态-反健态 自旋-轨道     

准二维无序系统的电子结构

对形如N_t×N_l型准二维无序系统，只考虑格点之间的最近邻跳跃积分，采用特殊的格点编号方案，在单电子近似下，系统的哈密顿量可表示为简明对称矩阵，借助豪斯荷尔德变换将其约化为对称三对角矩阵，再利用负本征值理论及传输矩阵等方法,对系统态密度、局域长度及电导等电子结构特性进行数值计算. 重点研究了准一维四平行链和五平行链无序系统， 将结果与一维单链、准一维双链及三链系统进行对比，发现随维度的增加，系统的能带有所展宽，能态密度分布发生很大的变化，其峰值数量呈偶数规律增加. 并且在能带中心处存在有局域长度大于系统大小的扩展态，处于这些态下的系统具有较大电导. 从单链到多链，相当于扩大了系统的关联范围，使系统出现了类似非对角长程关联的行为. 关键词： 准二维无序系统 态密度 局域长度 电导     

磁场中的拓扑绝缘体边缘态性质

用数值方法研究了拓扑绝缘体薄膜体系在外加垂直磁场 作用下其边缘态的性质. 磁场的加入通过耦合k+eA, 即Peierls势替换关系和 该作用导致的Zeeman交换场体现在哈密顿量中. 考虑窄条圆环状结构的二维InAs/GaSb/AlSb薄膜量子阱材料, 当其处于拓扑非平庸状态, 即量子自旋霍尔态时, 会出现受时间反演对称性保护的两支简并边缘态, 而在垂直磁场的作用下, 时间反演对称性被破坏, 这时能带将形成一条条的朗道能级, 原来简并的两支边缘态也会分开到朗道能级谱线的两侧, 从电子态密度的空间分布情况则可以看到边缘态分别局域在材料的两个边界. 随着磁场的增大, 位于同一边界上的不同 自旋极化的边缘态将出现分离: 一支仍然局域在边缘, 另一支则随外加磁场的增加而有逐渐演化到材料内部的趋势. 文中还计算了同一边界上的两支边缘态之间的散射, 结果表明由于两个边缘态在空间发生分离, 相互之间的散射被很大的压制, 得到了其散射随磁场增加没有明显变化的结论, 所以磁场并不会增强散射过程, 也没有破坏体拓扑材料的性质, 说明了量子自旋霍尔态在没有时间反演对称的情况下也可以有较强的稳定性.     

ZnLaxSn-x（x=4，8）半导体材料光电性质的第一性原理计算

本文采用第一性原理研究了ZnS,Zn La4Sn-4,ZnLa8Sn-8三个体系的能带结构、分态密度、总态密度以及复介电函数.研究结果显示: ZnS体系的价带顶和导带底都在G点处,为直接带隙材料;ZnLa4Sn-4,Zn La8Sn-8两体系的导带向价带移动,费米能级处有多条能级穿过,费米能级处附近的态密度则由S 3p ,Zn 3p和La 5d以及5p 轨道组成;对比分析ZnS,ZnLa4Sn-4,ZnLa8Sn-8三个体系的复介电函数的虚部发现Zn La4Sn-4、ZnLa8Sn-8体系在0-10eV区域内的介电峰都少于本征结构,而在10到20eV的高能量区间内,ZnLa4Sn-4、ZnLa8Sn-8体系介电峰的相应幅度较ZnS增强.     

Effects of bias on dynamics of an AC-driven two-electron quantum-dot molecule

The effects of bias on the dynamical localization of two interacting electrons in a pair of coupled quantum dots driven by external AC fields have been numerically investigated. With an effective two-site model and Floquet formalism,the time-dependent Schroedinger equation is numerically solved and the Pmin, the minimum of the population evolution of the initial state within a certain time period, is used to quantify the degree of the dynamical localization. Results indicate that the bias can change the energy of the initial state and break the dynamical symmetry of the system with a pure AC field. And the amplitude of the AC field with dynamical localization phenomenon changes with bias. All the numerical results are explained by the perturbation theory and two-level approximation.     

Dynamical localization effect in a coupled quantum dot array driven by an AC magnetic field

We have studied the transport properties of a ring-coupled quantum dot array driven by an AC magnetic field, which is connected to two leads, and we give the response of the transport current to the dynamical localization. We found that when the ratio of the magnetic flux to the total quantum dots number is a root of the zeroth order Bessel function, dynamical localization and collapse of quasi-energy occurs and importantly, the transport current displays a dip which is the signal of dynamical localization. The dynamical localization effect is strengthened as a result of the increase of the quantum dot number, and it is weakened on account of the increase of the dots-lead hopping rate.     

Dynamical electron localization and self-induced transparency in semiconductor superlattices

The mechanisms of the occurrence of self-induced and selective transparencies of semiconductor superlattices in a strong time-dependent electric field are investigated. The association of these mechanisms with Bloch oscillations, dynamical localization, and collapse of electron quasi-energy minibands is analyzed, and a comparison with the properties of Josephson junctions is made. It is shown that the self-induced transparency is due to the fact that the current-contributing component of the electron distribution function is destroyed by collisions at discrete values of the amplitude of the time-harmonic field, while the selective transparency is associated with the nonmonotonic dependence of the spectrum of nonlinear electron oscillations in the electric field on the amplitude of the field. The dynamical localization and collapse of quasi-energy minibands lead to the field energy dissipation and are favorable to destruction of the transparency states of the superlattice.     

Dynamical properties of two-band superlattices with strong interband coupling in real space under the action of ac and dc－ac fields

Within a two-band tight-binding model driven by ac and dc－ac electric fields, using numerical methods, we investigate the dynamics of electrons and the quasi-energy spectrum of the system with strong interband coupling in real space. We find that when the bandwidth is suppressed to a value much smaller than the field frequency, the dynamical localization can exist in the system. The corresponding regions are found for the occurrence of dynamical localization in the parameter space.     

Dynamical behaviors of an exciton in an asymmetric double coupled quantum dot

Dynamical behaviors of an exciton in an asymmetric double coupled quantum dot and an alternatingcurrent (ac) electric field have been analyzed based on the two-level approximation theory, and the conditions under which dynamical localization occurs are obtained. It shows that when the amplitude of the ac electric field is small, the Coulomb interaction plays an important role. The dynamical behaviors of the exciton are mainly confined in the low-level subspace. When the ratio of the field intensity to frequency is the root of Bessel function, electron and hole are localized in one dot, and they can be divided with the increasing amplitude of the ac electric field. __________ Translated from Chinese Journal of Semiconductors, 2005, 26(6) (in Chinese)     

Intersections of quasi-energy levels in the relativistic theory of multiply charged ions

The relativistic generalization of the quasi-energy method is given with the help of which the influence of an alternating electric field on the energy levels (2 $\text{1}\text{2}$ 0), (2 $\text{1}\text{2}$ 1) of multiply charged H-like ions is investigated. Intersections are found of quasi-energy levels in external fields with frequencies ω₀ ? ΔE_L and definite values of the field amplitude.     

The dynamic Casimir effect within a vibrating metal photonic crystal

The lattice-vibrating metal photonic crystal is exactly a system of dynamical Casimir effect connected in series, and so we can expect that a dynamical Casimir effect is enhanced by the photonic band effect. In the present study, when an electromagnetic field between metal plates is in the ground state in a one-dimensional metal photonic crystal, the radiation of electromagnetic wave in excited states has been investigated by artificially introducing lattice vibration to the photonic crystal. In this case as well as a dynamical Casimir effect, it has been shown that the harmonics of a ground state are generated just by vibrating a photonic crystal even without an incident wave. The dependencies of the radiating power on the number of layers and on the wavenumber of the lattice vibration are remarkable. It has been found that the radiation amplitude on lower excited states is not necessarily large and radiation on specific excited levels is large.     

Dynamical Localization in Disordered Quantum Spin Systems

We say that a quantum spin system is dynamically localized if the time-evolution of local observables satisfies a zero-velocity Lieb-Robinson bound. In terms of this definition we have the following main results: First, for general systems with short range interactions, dynamical localization implies exponential decay of ground state correlations, up to an explicit correction. Second, the dynamical localization of random xy spin chains can be reduced to dynamical localization of an effective one-particle Hamiltonian. In particular, the isotropic xy chain in random exterior magnetic field is dynamically localized.