Optische Übergänge in hochdotierten und gemischten Halbleitern: Die verallgemeinerte Bloch-Matrix

To treat fluctuating perturbation potentials in semiconductors, it is necessary to take quantum corrections to the Thomas-Fermi approximation into account. A generalization of the Bloch matrix, valid for arbitrary degrees of degeneracy, is used to handle these quantum corrections. The generalized Bloch equation and the results of a treatment of optical transitions in high-doped semiconductors with this method are given. The concept of effective band edges is shown to be useful for the problem on hand.     

Quantum corrections to the electrical conduction in an AlGaN/GaN heterostructure

A new method for magneto-transport characterisation of semiconductor heterostructures is presented. The classical model of mixed conduction, modified by corrections resulting from quantum effects, has been used in the analysis of the conductivity-tensor components, magnetoresistance, and Hall coefficient in n-type Al_0.85Ga_0.15N/GaN in magnetic fields up to 12 T, in the temperature range from 2 to 295 K. The mixed conduction is due to high-mobility carriers in the conduction band in the interface and to low-mobility carriers in the conduction band in the GaN layer and in an impurity band. The corrections to the conduction of high-mobility carriers result from quantum effects: negative magnetoresistance, extraordinary Hall effect, and freeze-out of electrons. Negative magnetoresistance is due to localisation of electrons and to increasing tunnel coupling between electron states in different minima of a random potential, due to interface roughness. The extraordinary Hall effect has been explained by interaction of electrons with magnetic moments of dislocations in the interface. Decreasing concentration of electrons is probably due to Landau quantisation of the conduction band in the interface of the heterostructure. Received: 27 November 2000 / Accepted: 18 December 2000 / Published online: 3 April 2001     

p型掺杂Si/Ge量子点的电子拉曼散射(英文)

在本文中我们首次报道了p型掺杂的自组织Si/Ge量子点中空穴能级子带间的电子拉曼散射,此电子跃迁的能量为105meV。Si/Ge量子点Ge Ge模的共振拉曼散射表明此空穴能级间的电子拉曼散射与Γ点附近的E0(≈2.52eV)发生了共振,而E1的能量小于2.3eV.变温实验和偏振实验进一步证实了我们的指认。所有观测的实验数据与6 bandk·p能带结构理论的计算结果吻合得很好。     

Electron localization in a nondegenerate semiconductor with a random potential due to charged impurities

The saturation of the electron mobility, as determined according to the magnetoresistance, was observed in a semiconductor with a large-scale potential due to charged impurities. It was shown that the saturation is due to the existence of a quantum mobility threshold. A negative magnetoresistance of nondegenerate electrons, which is due to the suppression of quantum interferences corrections to the conductivity by the magnetic field, was found. The magnitude of these effects near the mobility threshold was explained by the absence of short, closed, electronic trajectories in the large-scale potential. A relation was established between the amplitude of the random potential and the saturated values of the mobility and the quantum corrections to the conductivity.     

Entanglement and excited-state quantum phase transition in an extended Dicke model

We investigate the properties of entanglement and excited-state quantum phase transition (ESQPT) in a hybrid atom-optomechanical system in which an optomechanical quadratic interaction is introduced into a normal Dicke model. Interestingly, by preparing the ancillary mode in a coherent state, both the quantum entanglement and ESQPT can be realized in a relative weak-coupling condition. Moreover, the entanglement is immune to the A² term, and a reversed trend of the entropy is obtained when the A² term is included. Density of states (DoS) and Peres lattice are used to investigate ESQPTs. Compared to a normal Dicke model, the DoS enlarges exp(2r_α) times if the ancillary mode is prepared in a coherent state. This work is an extension of the ground-state quantum phase transition, which may inspire further exploration of the quantum criticality in many-body systems.     

硅量子点的形状及其弯曲表面效应

硅量子点的弯曲表面引起系统的对称性破缺, 致使某些表面键合在能带的带隙中形成局域电子态.计算结果表明:硅量子点的表面曲率不同形成的表面键合结合能和电子态分布明显不同. 例如, Si–O–Si桥键在曲率较大的表面键合能够在带隙中形成局域能级, 而在硅量子点曲率较小的近平台表面上键合不会形成任何局域态, 但此时的键合结合能较低. 用弯曲表面效应(CS)可以解释较小硅量子点的光致荧光光谱的红移现象. CS效应揭示了纳米物理中又一奇妙的特性. 实验证实, CS效应在带隙中形成的局域能级可以激活硅量子点发光. 关键词： 硅量子点 弯曲表面效应 表面键合 局域能级     

Structural phase transition of vanadium at 69 GPa

A phase transition was observed at 63-69 GPa and room temperature in vanadium with synchrotron x-ray diffraction. The transition is characterized as a rhombohedral lattice distortion of the body-centered-cubic vanadium without a discontinuity in the pressure-volume data, thus representing a novel type of transition that has never been observed in elements. Instead of driven by the conventional s-d electronic transition mechanism, the phase transition could be associated with the softening of C44 trigonal elasticity tensor that originates from the combination of Fermi surface nesting, band Jahn-Teller distortion, and electronic topological transition.     

Self-consistent electronic structure of quantum wells by invariant embedding method

A new approach based on the invariant embedding method for the self-consistent calculation of electronic structure of quantum wells is presented and is applied to both neutral quantum well and parabolic quantum well. Numerical results obtained for these structures agree very well with those of previous theoretical and experiment studies. The present approach is expected to lead to a more efficient and stable scheme for the calculation of electronic band structure of quantum structures. Realistic boundary conditions are naturally taken into account in the present calculation which provides a convenient way for studying boundary effects. Received 21 September     

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp²hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.     

An effective quantum parameter for strongly correlated metallic ferromagnets

The correlated motion of electrons in metallic ferromagnets is investigated in terms of a realistic interacting-electron model with N-fold orbital degeneracy and intra-orbital (U) and inter-orbital (J) Coulomb interactions. Correlation-induced self-energy and vertex corrections are incorporated systematically to provide a non-perturbative Goldstone-mode-preserving scheme. An effective quantum parameter [U2+(N-1)J2]/[U+(N-1)J]2 is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large N, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. Correlation effects are investigated for spin stiffness, magnon dispersion, electronic spectral function, density of states, and finite-temperature spin dynamics using realistic bandwidth, interaction, and lattice parameters for iron.     

Spin polarization of quantum well states in Ag films induced by the Rashba effect at the surface

The electronic structure of Ag(111) quantum well films covered with a (sqrt[3]xsqrt[3]) R30 degrees Bi/Ag surface ordered alloy, which shows a Rashba spin-split surface state, is investigated with angle-resolved photoemission spectroscopy. The band dispersion of the spin-split surface state is significantly modified by the interaction with the quantum well states of Ag films. The interaction is well described by the band hybridization model, which concludes the spin polarization of the quantum well states.     

Rashba spin-orbit splitting energy of a pressure induced hydrogenic donor impurity in a corrugated quantum well

Pressure induced hydrogenic donor impurity of ground and excited state in a GaAlAs/GaAs/GaAlAs corrugated quantum well is investigated. The calculations have been carried out using variational technique within the single band effective mass approximation taking into account the anisotropy and the corrections due to the conduction band nonparabolicity. The energy dependent effective mass and the position dependent quantity (Rashba spin-orbit splitting energy) are introduced to obtain the binding energy as a function of well width in the influence of pressure. The obtained results are compared with the other existing literature available.     

Moment analysis and quantum effects in collision-induced absorption

In this work three moments of collison-induced absorption spectra which include lowest order quantum corrections are used to obtain information about the induced dipole moment and about the role of quantum effects in collision-induced absorption. The two experimental cases discussed are the induced translational band of a He-Ar mixture at 295 K and the induced fundamental band of a pH₂-Ar mixture at 160 K. In both cases the quantum corrections to a classical moment analysis were found to be significantly greater than the experimental error and cannot be ignored. Using several models for the dependence of the induced dipole moment on intermolecular separation, the form of the induced dipole is found for the small range of intermolecular separation (2·5 Å–3·2 Å) from which most of the absorption was found to arise.     

SIZE AND SHAPE DEPENDENCE OF THE LOWEST CONDUCTION BAND STATES IN GaAs QUANTUM DOTS

Using a recently developed new scheme, we investigated the electronic structure of the lowest conduction band states of cuboid GaAs quantum dots with different shapes and sizes in a wide range from 4 to 120a (a is the lattice constant). The critical edge length of the direct/indirect crossover in GaAs cuboid quantum dots depends on the size and shape of the quantum dots. As a deduction we also discussed the critical size of dlrect/indirect transition in GaAs quantum wires and thin films.     

Nitride band-structure model in a quantum well laser simulator

The two-dimensional quantum well laser simulator Visible Laser Simulator (VLS) has been extended to simulate III–V nitride based lasers. In this paper, we present details about the laser simulator and focus on the six band k.p solver used to calculate the electronic band structure. The effect of strain on bulk and quantum well electronic dispersion is analyzed, and the influence of the built-in polarization charges on the band diagram is likewise considered.     

Ce 基重费米子材料的电子态研究与维度调控

重费米子材料作为一类典型的强关联电子体系,蕴含着非常规超导、奇异金属、量子临界、 磁有序、重电子态、关联拓扑态等新奇的量子态,而4f 电子在其中扮演着重要的作用。随着高分 辨角分辨光电子能谱和薄膜生长技术的发展,精确探测重费米子材料中4f 电子在能量/动量空间 的色散和谱权重成为了可能,这为从微观上理解这类材料中的电子关联效应和新奇量子现象提供 了重要的基础。本论文总结了几个典型的重费米子单晶和薄膜体系的电子态研究,包括Ce-115 体 系、CeCu2Si2、CeRh6Ge4 以及单晶 Ce 膜等。这些结果为理解重费米子体系中重电子态的形成 和温度演化、近藤杂化的能带/动量依赖、重电子能带与超导的关系、近藤效应与磁性和其它量子 态的竞争、4f 电子的维度调控等重要物理问题提供了谱学证据。     

Generation of Fabry–Pérot oscillations and Dirac state in two-dimensional topological insulators by gate voltage

We investigate electron transport through Hg Te ribbons embedded by strip-shape gate voltage through using a nonequilibrium Green function technique. The numerical calculations show that as the gate voltage is increased, an edgerelated state in the valence band structure of the system shifts upwards, then hangs inside the band gap and merges into the conduction band finally. It is interesting that as the gate voltage is increased continuously, another edge-related state in the valence band also shifts upwards in the small-k region and contacts the previous one to form a Dirac cone in the band structure. Meanwhile in this process, the conductance spectrum displays as multiple resonance peaks characterized by some strong antiresonance valleys in the band gap, then behaves as Fabry–P′erot oscillations and finally develops into a nearly perfect quantum plateau with a value of 2e~2/h. These results give a physical picture to understand the formation process of the Dirac state driven by the gate voltage and provide a route to achieving particular quantum oscillations of the electronic transport in nanodevices.     

InGaN量子阱的微观特性

采用VASP程序包模拟计算InGaN量子阱的能带,精细展示了量子阱实空间能带结构。计算结果表明,In原子所在区域出现局域束缚态,导带底与价带顶的简并能级发生分裂,同时量子阱沿垂直结面方向存在分立的能级。此外,针对影响能带的In组分波动、能带弯曲等问题进行探讨,以准确描述其电子行为,从而深入系统地了解InGaN/GaN量子阱的电学光学等特性。     

Calculation of optical properties of a quantum dot embedded in a GaN/AlGaN nanocolumn

The TiberCAD simulation tool for calculation of optical and electronic properties of nanostructured devices has been used to study spontaneous emission of a GaN quantum dot embedded in an AlGaN nanocolumn. Macroscopic calculations provide corrections to the quantum calculation, showing the role of strain and the polarization field in spectra and the electron and hole states arrangement.     

Intrinsic spin hall effect induced by quantum phase transition in HgCdTe quantum wells

The spin Hall effect can be induced by both extrinsic impurity scattering and intrinsic spin-orbit coupling in the electronic structure. The HgTe/CdTe quantum well has a quantum phase transition where the electronic structure changes from normal to inverted. We show that the intrinsic spin Hall effect of the conduction band vanishes on the normal side, while it is finite on the inverted side. By tuning the Cd content, the well width, or the bias electric field across the quantum well, the intrinsic spin Hall effect can be switched on or off and tuned into resonance under experimentally accessible conditions.