近藤共振现象及其在低维电子系统中的实现

文章全面、系统地介绍了近藤效应、近藤问题、近藤共振现象的起源和研究历史的发展过程,提供了一个清晰而准确的近藤物理问题的图像.同时,文章还讨论了近年来近藤共振现象在各种低维电子关联系统中的实现.     

近藤共振电子结构的蜃影

海市蜃楼现象源于光在非均匀媒质内部被连续不断地折射 .如果地面上方的空气密度随高度的增加而减小 ,并形成稳定的分布 ,那么原本位于前方的景物看起来会上移 ,形成“空中楼阁” ,即所谓“上蜃” .蜃景的形成完全可以从光的经典波动性质得到解释 .不仅光 ,声波也可能聚焦投影成像 .一个人在天坛回音壁内的某个位置击掌 ,另一个人所听到的信号将依赖于他所处的位置 :可能听到一声 ,两声 ,甚至三声 ;也可能什么都听不到 .这里的机制比海市蜃楼的形成要复杂一些 ,不仅要考虑回音壁对声波的反射 ,而且需要计算从回音壁不同位置反射的分波的相干…     

低维超导材料中的量子振荡现象

低维超导材料由于具有尺度接近量子临界尺寸的优势,能够观测到显著的超导量子振荡效应,因此成为研究超导量子振荡效应的优异平台.由于这些量子振荡效应的周期、振幅、相位与磁通涡旋的量子化及运动方式、超导电子的配对机制、特定外部条件下超导体中的涨落和激发现象密切相关,并且它们还能直观地反映超导材料的几何结构对其超导物性的影响,因此对低维超导体中振荡效应的研究直接反映了超导体的本质规律,成为研究材料超导机制的一种重要手段,有着深邃的物理内涵和丰富的研究价值.本文将探讨三类能够在低维超导材料中观测到的典型超导量子振荡效应:利特尔-帕克斯效应、磁通涡旋运动导致的振荡效应和韦伯阻塞效应,从研究手段、理论预期、实验现象以及实验结果诸方面综述其中所揭示的深刻物理规律,并展望低维超导体的量子振荡效应在量子计算、器件物理和低温物理等领域的应用价值.     

非对角无序和维数效应对低维无序系统电子结构的影响

运用负本征值理论，探讨了非对角无序、维数效应对低维无序系统电子结构的影响，研究表 明，非对角无序和维数效应对低维无序系统电子结构的影响很大.非对角无序主要体现出系 统的结构变化和粒子边界效应；从一维单链、准一维双链到准一维三链无序系统，电子局域 化程度加大，电子能带结构更复杂，体现出显著的维数效应. 关键词： 电子结构 低维无序系统 非对角无序 维数效应     

串型耦合双量子点之间库仑作用对其近藤共振的影响

从理论上研究了串型耦合双量子点之间库仑作用对其近藤共振的影响. 采用非平衡态格林函数和奴役玻色子平均场近似方法求解了系统的哈密顿量; 计算了系统电子的态密度、透射率、占居数和近藤温度随双量子点之间库仑作用能的变化, 同时也计算了电极处于极化时双量子点之间库仑作用能对系统电子态密度的影响. 结果表明,双量子点之间库仑作用能够极大地影响系统的基态物理性质. 同时还对相关的物理问题进行了讨论.     

弱噪声极限下二维布朗运动的随机共振现象

在弱噪声极限下，应用多维Fokker-Planck方程的与时间有关问题方面的理论，推导了对称双稳势阱中二维布朗粒子的概率跃迁速率，并基于绝热近似理论研究了弱噪声极限下受小幅低频周期力驱动的二维布朗运动的随机共振现象.与以前的结果相比，所得到的信噪比与阻尼系数的大小无关.精确的数值模拟结果进一步说明了所得的信噪比的适用性. 关键词： 二维布朗运动 弱噪声极限 随机共振 信噪比     

半导体低维体系简介

介绍了量子阱、量子线、量子点、半导体超晶格、二维电子气等典型的低维半导体结构及其性质和分析方法。     

吸附位置对分子近藤效应的影响

文章报道了利用低温扫描隧道显微镜观察到磁性分子酞菁铁(iron phthalocyanine,FePc)在金属表面Au(111)上的近藤效应(Kondo effect).从实验上观察到两种由近藤效应引起的不同的Fano共振现象.通过理论模拟和分析,文章作者发现这两种Fano共振分别对应于不同的分子吸附位置.吸附位置不仅影响Fano共振的线型,而且还影响自旋电子的耦合强度,是调控金属表面磁性杂质自旋态的重要途径.     

dμz／dt=0在电子自旋共振中的物理含义

具有净自旋磁矩物质中的电子在ｚ轴方向外磁 场的作用下，ｄμｚ／ｄｔ＝０从而导致μｚ＝Ｃ，Ｃ为正、负 常数和零，本文对此进行了理论分析和讨论．结果表明：Ｃ 取值的不同，标志着在ｚ轴所加的磁场不同，但只要满足 在垂直于ｚ轴方向上射频场（微波场）的旋转方向和频 率与自旋磁矩μ的拉莫尔进动的方向和频率相同的条 件，都可发生电子自旋共振．并给出了μｚ取不同常数 时μ绕磁场的拉莫尔进动轨迹．     

Variational local moment approach: From Kondo effect to Mott transition in correlated electron systems

The variational local moment approach (VLMA) solution of the single impurity Anderson model is presented. It generalizes the local moment approach of Logan et al. by invoking the variational principle to determine the lengths of local moments and orbital occupancies. We show that VLMA is a comprehensive, conserving and thermodynamically consistent approximation and treats both Fermi and non-Fermi liquid regimes as well as local moment phases on equal footing. We tested VLMA on selected problems. We solved the single- and multi-orbital impurity Anderson model in various regions of parameters, where different types of Kondo effects occur. The application of VLMA as an impurity solver of the dynamical mean-field theory, used to solve the multi-orbital Hubbard model, is also addressed.     

Kondo impurities in nanoscopic and mesoscopic systems

We present results for Kondo impurities in nanoscopic systems. Using Wilson's numerical renormalization group we analyze two different situations: an isolated system with a discrete spectrum of well-defined energy levels and a fixed number of electrons and a nanoscopic system weakly coupled to a macroscopic reservoir. In the latter case, new regimes not observed in macroscopic homogeneous systems are induced by the confinement of conduction electrons. These new confinement-induced regimes are very different depending on whether the Fermi energy is at resonance or between two quasi-bound states.     

Heat transport in low-dimensional systems

Recent results on theoretical studies of heat conduction in low-dimensional systems are presented. These studies are on simple, yet non-trivial, models. Most of these are classical systems, but some quantum-mechanical work is also reported. Much of the work has been on lattice models corresponding to phononic systems, and some on hard-particle and hard-disc systems. A recently developed approach, using generalized Langevin equations and phonon Green's functions, is explained and several applications to harmonic systems are given. For interacting systems, various analytic approaches based on the Green–Kubo formula are described, and their predictions are compared with the latest results from simulation. These results indicate that for momentum-conserving systems, transport is anomalous in one and two dimensions, and the thermal conductivity κ diverges with system size L as κ ～ L ^α. For one-dimensional interacting systems there is strong numerical evidence for a universal exponent α = 1/3, but there is no exact proof for this so far. A brief discussion of some of the experiments on heat conduction in nanowires and nanotubes is also given.     

de Haas–van Alphen effect in strongly correlated electron systems

We present the Fermi surface properties in strongly correlated electron systems of rare earth and uranium compounds via de Haas–van Alphen experiments. The conduction electrons with large cyclotron effective masses over 100m₀ (m₀: rest mass of an electron) are detected in CeRu₂Si₂, CeCoIn₅ and UPt₃. These electrons move slowly in the crystal. The topology of the Fermi surface and the cyclotron mass are compared to those of energy band calculations.     

RIXS in correlated electron systems under extreme conditions

ABSTRACT

We report here on the application of Resonant Inelastic X-ray Scattering (RIXS) in correlated electrons systems under pressure. Thanks to its bulk sensitivity and superior resolving power, RIXS appears as a powerful spectroscopic technique to unravel the local electronic and magnetic properties of materials at extreme conditions. The method is illustrated in vanadium-oxides- and Fe-based superconductors at high pressure.     

Asymmetric conductivity of the Kondo effect in cold atomic systems

Motivated by recent theoretical and experimental advances in quantum simulations using alkaline earth(AE)atoms,we put forward a proposal to detect the Kondo physics in a cold atomic system.It has been demonstrated that the intrinsic spin-exchange interaction in AE atoms can be significantly enhanced near a confinement-induced resonance(CIR),which facilitates the simulation of Kondo physics.Since the Kondo effect appears only for antiferromagnetic coupling,we find that the conductivity of such system exhibits an asymmetry across a resonance of spin-exchange interaction.The asymmetric conductivity can serve as the smoking gun evidence for Kondo physics in the cold atom context.When an extra magnetic field ramps up,the spin-exchange process near Fermi surface is suppressed by Zee-man energy and the conductivity becomes more and more symmetric.Our results can be verified in the current experimental setup.     

Disorder-induced local magnetic moments in weakly correlated metallic systems

We consider the formation and the Kondo effect of local magnetic moments in the Anderson-Hubbard model with off-diagonal disorder. The existence of moments at sites weakly coupled to the environment is deduced in effective medium approximation. The distribution of moments is calculated both deep in the metallic phase and near the metal-insulator transition. We discuss the Kondo quenching of the moments and derive a distribution of local Kondo temperatures.     

关联电子体系中的轨道物理

轨道自由度为凝聚态关联电子材料带来丰富多彩的新量子相的同时也导致了复杂性,对理解强关联电子体系的本质带来了挑战。文章系统地介绍了多轨道关联电子体系的各种物理性质。首先简要地介绍了轨道自由度在过渡金属氧化物和稀土化合物中的重要作用和轨道序的物理图象;其次论述了轨道极化、轨道序及其与晶格畸变的联系;然后讨论了轨道序的理论研究及其在实验上的可能探测;接着介绍了多轨道体系中的金属—绝缘体相变和多轨道超导电性的新特征;最后简短讨论了目前轨道物理研究中面临的问题和挑战。     

Superconductivity in weakly correlated electron systems

We study the influence of the short-ranged Hubbard correlation U between the conduction electrons on the Cooper pair formation in normal (s-wave) superconductors. The Coulomb correlation is considered within the standard second order perturbation theory, which becomes exact in the weak coupling limit but goes beyond the simple Hartree-Fock treatment by yielding a finite lifetime of the quasiparticles at finite temperature. An attractive pairing interaction V, which may be mediated by the standard electron-phonon mechanism, is considered between nearest neighbor sites. A critical value for the attractive interaction is required to obtain a superconducting state. For finite temperature a gapless superconductivity is obtained due to the finite lifetime of the quasiparticles, i.e. the Coulomb correlation has a pair-breaking influence. The energy gap and depend very sensitively on U, V and band filling n and develop a maximum away from half filling as function of n. The ratio varies with n, being higher than the BCS value near half filling and reaching the BCS value for lower n. Received 17 February 1999