由超辐射引起的迁移率边和重返局域化

研究了束缚在梯子形光晶格中的中性原子与高精度腔耦合的系统,发现由超辐射引起的准周期调制可以导致迁移率边和重返局域化现象的出现.在平均场近似下,超辐射现象可以引起两种不同频率的准周期调制,它们可以由腔场和泵浦场有效调节.在本文的观测范围内,当高频调制强度小于某一临界值时,系统随着低频调制强度的增加经历一次局域转变.通过数值求解分形维度、密度分布、平均参与率、平均逆参与率以及标度分析,证明了局域转变历经的临界相区存在迁移率边;当高频调制强度大于临界值时,随着低频调制强度的增强,系统依次经历完全扩展相-临界相-完全局域相-临界相-完全局域相,这是一个典型的重返局域化现象.最后给出了局域化相图.该研究结果为超辐射相变和重返局域化现象的研究建立了联系,也为重返局域化的研究搭建了新的平台.     

无序度对一维长程关联无序系统中局域化-退局域化转变的影响

对长程幂律关联能量序列进行了修正,使其能体现出无序度在一维长程关联无序系统中的影响,并利用重正化群方法,计算了能反映该系统局域化-退局域化转变的Lyapunov指数.结果表明,在由于关联指数p的影响而在系统中出现的局域化向退局域化的转变中,无序度起着相反的作用.当关联指数p一定而无序度W增大时,系统中心能区范围内由于长程关联而引起的扩展态逐渐向局域态转变.当无序度W增大到某一临界值W_c时,系统中所有本征态均转变为局 关键词： 长程关联 Lyapunov指数 无序度 局域化-退局域化转变     

强非局域光晶格中空间孤子的脉动传播

根据强非局域结构中空间孤子的演化方程——非局域非线性薛定谔方程,采用分步傅里叶方法,对一维强非局域光晶格结构中空间孤子的脉动传播进行数值研究.分析了孤子的初始光束能量、非局域程度、光晶格调制强度、光晶格周期以及线性折射率的纵向调制率与空间孤子的脉动传播周期之间的关系,并对影响脉动传播的内在物理机制进行了讨论.同时,还研究了在线性折射率纵向调制情况下强非局域光晶格结构中空间孤子传输的开关行为. 关键词： 非局域非线性薛定谔方程 光晶格 空间孤子 光开关     

非均匀无序系统的声子局域化

本文根据电、声子类比关系,由重整化群方法在电子局域化问题上的应用结果,给出了非均匀无序系统的声子局域化信息。发现非均匀无序系统的声子频谱上可存在三条迁移率边。 关键词：     

非完整超晶格中电子透射问题的计算机模拟

采用转移矩阵方法,模拟研究了垒高无序和阱宽无序非完整超晶格的电子态问题.计算了垒高无序有限超晶格的透射谱和其局域态波函数以及阱宽无序有限超晶格的透射谱和本征值,直观地给出了垒高无序和阱宽无序非完整有限超晶格其电子态行为的物理图像.模拟结果表明:垒高无序和阱宽无序这两种常见非完整一维有限超晶格的子带带隙间均存在强烈的电子运动定域化,且电子波的布喇格散射对周期性势场更敏感;这两种非完整性引起的局域,通过计算电子局域态波函数和有限系统的本征值得到了证实;对本文讨论的这种类型和周期的超晶格,如果控制阱宽在9.1～10.9nm间随机变化,即阱宽的值最大相差1.8岫时,计算机模拟的结果是,阱宽的这种非周期性开始使子带的带隙消失.     

无序层状系统电导率的自洽研究

本文运用无序层状系统模型模拟无序超晶格的性质,根据β-S方程,在弱局域限,给出了动力学密度响应函数x(q,ω),并由x(q,ω)求得了平行和垂直于平面的电导率和局域化长度的表达式,导出了依赖于频率ω的扩散系数D_k(ω)的自洽方程,并进行了讨论。 关键词：     

电声子相互作用对半导体超晶格动力学局域化的影响

本文用单带模型分别研究了直流外场和交流外场作用下，电声子相互作用对半导体超晶格的动力学局域化性质的影响。结果表明：电声子相互作用会破坏电子的动力学局域化。对同一声子频率，电声子相互作用的强度越大，电子越快被散射；当声子频率等于直流场的布洛赫频率或交流场频率时，初始局域在某一格点的电子被迅速地散射到其他格点上去，亦即光声子共振场会严重破坏电子的动力学局域化性质。     

准周期调制下自旋1/2反铁磁XY模型中的晶格畸变行为

利用严格对角化方法研究了Thue-Morse准周期调制下 自旋1/2反铁磁XY模型中的晶格畸变行为. 结果显示: 系统中每个格点的晶格畸变幅度介于均匀分布和无序分布之间. 对于较弱的准周期调制, 调制强度的增加有利于晶格畸变的形成. 但是, 对于较强的准周期调制, 调制强度的增加则阻碍晶格畸变的形成. 此外, 系统低能谱的能隙也明显受到准周期调制的影响. 关键词： 晶格畸变 Thue-Morse序列 准周期调制     

非晶光子晶体中的光子局域化

利用多重散射方法计算并研究了二维光子晶体随着无序度变化的光子局域化.通过控制方形单元随机旋转角度以控制光子晶体的无序度.研究发现，随着无序度的增加光子通带的透过率逐渐降低，而光子禁带中的透过率逐渐上升，即无序导致的局域化逐渐由光子带边向光子禁带中心和光子通带的中心扩展.而且光子通带中的平均透过率随无序度的增加呈e指数下降. 关键词： 非晶 光子晶体 无序 光子局域     

混合价锰氧化物的金属—绝缘体转变和特大磁电阻理论

提出了一个包括双交换自旋无序和非磁无序的局域化模型，解释了锰氧化物的异常磁性和输运性质．     

Delocalization of quasiparticles in quasi-two-dimensional disordered superconductors with s-wave pairing

We investigate localization behavior of quasiparticles in disordered multi-plane superconductors with s-wave pairing. By introducing disorder with random site energies, the spatial fluctuations of Bogoliubov-de Gennes pairing potential are self-consistently determined. The size dependence of rescaled localization length for a long bar is calculated by using the transfer-matrix method. From the finite-size scaling analysis we show that there exists a critical point of the disorder strength W_c which separates the extended and localized quasiparticle states in such quasi-two-dimensional systems. The associated critical behavior is studied and the relationship of the results to the number of planes is discussed.     

The scaling theory of electrons in disordered solids: Additional numerical results

We present extensive numerical results applying the finite size scaling method to the theory of electrons in disordered systems. A method is developed for studying the localisation length in 1-dimensional systems of finite cross section. By studying these results as a function of cross-section and using scaling ideas, we derive the critical properties of 2-D and 3-D systems. We calculate transport properties as a function of temperature which can then be compared with experiment.     

Anderson localization for the diamond lattice studied by a real-space renormalization

The problem of Anderson localization for strongly disordered electronic systems on a diamond lattice is studied by a real-space renormalization for a very large system of 27,000 sites. The renormalization, which is exact in principle, is based on the transformation of the system considered into an equivalent chain system. The mobility edges as a function of the strength of disorder and the critical value for the Anderson transition are calculated.     

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     

Entanglement entropy in a one-dimensional disordered interacting system: the role of localization

The properties of the entanglement entropy (EE) in one-dimensional disordered interacting systems are studied. Anderson localization leaves a clear signature on the average EE, as it saturates on the length scale exceeding the localization length. This is verified by numerically calculating the EE for an ensemble of disordered realizations using the density matrix renormalization group method. A heuristic expression describing the dependence of the EE on the localization length, which takes into account finite-size effects, is proposed. This is used to extract the localization length as a function of the interaction strength. The localization length dependence on the interaction fits nicely with the expectations.     

Localization length of wave functions in one-dimensional disordered systems with periodic modulation

The localization length of wave functions in one-dimensional disordered systems with periodic modulation is studied. The role of spatial inhomogeneity in the problem is considered. We calculate the localization length varying with energy for the compositionally modulated systems with either the disorder of Anderson distributions or the randomness of periods. The results show that the non-uniformity of disordered systems leads to some different properties of the localized wave functions.     

一维无序扰动周期结构中局域长度的对称等价变换

以一维周期结构光谱对称性为基础，提出了无序扰动周期结构有关局域长度的一个新的变换关系：对称等价变换，并用数值计算加以了验证.该等价变换描述了不同无序度的结构对不同频率光子局域能力之间的等价关系，为无序结构中光子局域性质的进一步研究提供了一个新的工具. 关键词： 局域长度 无序扰动周期结构 对称等价变换     

Electronic localization in disordered systems

A brief review is given of the current understanding of the electronic structure, transport properties and the nature of the electronic states in disordered systems. A simple explanation for the observed exponential behaviour in the density of states (Urbach tails) based on short-range Gaussian fluctuations is presented. The theory of Anderson localization in a disordered system is reviewed. Basic concepts, and the physics underlying the effects of weak localization, are discussed. The scaling as well as the self-consistent theory of localization are briefly reviewed. It is then argued that the problem of localization in a random potential within the so-called ladder approximation is formally equivalent to the problem of finding a bound state in a shallow potential well. Therefore all states are exponentially localized in d=1 and d=2. The fractal nature of the states is also discussed. Scaling properties in highly anisotropic systems are also discussed. A brief presentation of the recently observed metal-to-insulator transition in dequals;2 is given and, finally, a few remarks about interaction effects in disordered systems are presented.     

ON LOCALIZATION IN DISORDERED ELECTRON SYSTEMS

The field theory of Anderson localization of disordered electron systems is formulated in the framework of closed time path Green's functions (CTPGF). The properties of the mobility edge are studied by the renormalization group method and the density of states is found to be zero at the mobility edge.