磁场下含结构缺陷多组分超晶格中的局域电子态和电子输运

在有效质量近似理论下，利用转移矩阵和有效垒高方法研究了有限磁场下含结构缺陷的多组分超晶格中局域电子态的性质.在考虑各组分层有效质量的失配时，外加磁场会导致磁耦合效应的出现.磁耦合效应不仅引起局域电子能级的量子化，并且随着朗道指数或磁场强弱的变化，局域能级及其局域程度都会发生显著移动，特别是对高能区域的局域电子态影响更大.此外，还计算了电子输运系数，讨论了含结构缺陷的三组分超晶格中局域电子能级与输运谱透射禁区中的共振透射峰的关系，发现两者之间有着很好的对应关系，为相应的实验研究提供了依据. 关键词： 超晶格 局域电子态 磁场     

半导体与超晶格的数学模型及其分析Ⅰ:模型

本文论及半导体与超晶格的数学模型方法的一些新近发展动态。介绍了半导体与超晶格的一些背景材料并给出了半导体宏观模型间的层次框架。超晶格中载流子传输的SHE扩散模型也被观察。     

Ge_x Si_(1-x)/Si超晶格中纵声学声子的喇曼散射强度研究

本文用光弹理论,在全面考虑了超晶格中两种材料的声速,质量密度和光弹常数存在差别的基础上,计算了Ge_xSi_(1-x)/Si超晶格中折迭纵声学声子的喇曼散射强度,在高达50cm~(-1)的频率范围内,理论值和实验符合得很好。     

Mixed crystals in the system Cu2MnGexSn1−xS4: Phase analytical investigations and inspection of tetrahedra volumes

Cu₂MnGeS₄ crystallizes orthorhombic in a wurtzite superstructure type while Cu₂MnSnS₄ crystallizes in a tetragonal sphalerite superstructure type. Lattice constants and thermal analyses of the solid solution series Cu₂MnGe_xSn_1−xS₄ are presented. A two-phase region is found from Cu₂MnGe_0.3Sn_0.7S₄ to Cu₂MnGe_0.5Sn_0.5S₄. The cell volume of the mixed crystals increases with increasing Sn content. The melting points increase smoothly with increasing Ge content to x=0.5 and then steeply for higher Ge contents. The single crystal X-ray structure analysis of Cu₂MnGe_0.55Sn_0.45S₄ is presented. The refinement converges to R=0.0270 and wR₂=0.0586, Z is 2. The volumes of the tetrahedra [MS₄] (M=Cu, Mn, Ge, Sn) are calculated. From these volumes the differences in size of the tetrahedra are derived and compared with the corresponding differences in the end members of the solid solution series. It turns out that the resulting structure type in these materials depends on the volume differences of the constituting tetrahedra [MS₄].     

Surfaces and interfaces in short-period GaAs/AlAs superlattices

Photoelectron spectroscopy, in particular the angular resolved photoemission excited by ultraviolet radiation (ARUPS), provides the most direct experimental information about the electron structure of crystals, both of the bulk and of the low-index surfaces. The sensitivity of the method, as well as its difficulties, when applied to GaAs/AlAs superlattices are described. The new periodicity of these man-made crystals in the direction of their growth (e.g., in the layer-by-layer growth in molecular beam epitaxy), is responsible for opening of the new energy gaps (so-called minigaps) in the electron energy bands of crystals forming the superlattice. In addition to the well-known confinement of electrons at the valence and conduction band edges in long-period superlattices, the electron confinement to the interfaces has also been found in the vicinity of minigaps in short-period superlattices. The role of this confinement in the intensities of electrons photoemitted from superlattice surfaces is discussed. Superlattices with different thicknesses in the topmost layers represent systems with a simple change of the surface atomic structure. The predictions of one-dimensional models about a change of the surface-state energy within the band gap with a change of crystal potential termination are tested for the ideally terminated (1 0 0) surface of a very thin superlattice (GaAs)₂(AlAs)₂. The results of the energy distributions of photoemitted electrons, calculated in the one-step model of photoemission, show that the ARUPS experimental observation of surface-state shifts should be possible, at least in larger minigaps. The results indicate the possibility of a straightforward tuning of the electronic structure of the superlattice surface by geometrical means.     

On the two dimensional effective electron mass in quantum wells,inversion layers and NIPI superlattices of Kane type semiconductors in the presence of strong light waves: Simplified theory and relative comparison

An attempt is made to study the two dimensional (2D) effective electron mass (EEM) in quantum wells (Qws), inversion layers (ILs) and NIPI superlattices of Kane type semiconductors in the presence of strong external photoexcitation on the basis of a newly formulated electron dispersion laws within the framework of k.p. formalism. It has been found, taking InAs and InSb as examples, that the EEM in Qws, ILs and superlattices increases with increasing concentration, light intensity and wavelength of the incident light waves, respectively and the numerical magnitudes in each case is band structure dependent. The EEM in ILs is quantum number dependent exhibiting quantum jumps for specified values of the surface electric field and in NIPI superlattices; the same is the function of Fermi energy and the subband index characterizing such 2D structures. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the EEM varies in various manners with all the variables as evident from all the curves, the rates of variations totally depend on the specific dispersion relation of the particular 2D structure. Under certain limiting conditions, all the results as derived in this paper get transformed into well known formulas of the EEM and the electron statistics in the absence of external photo-excitation and thus confirming the compatibility test. The results of this paper find three applications in the field of microstructures.     

Extended phase diagram of La1-xCaxMnO3 by interfacial engineering

The interfacial enhanced ferromagnetism in maganite/ruthenate system is regarded as a promising path to broaden the potential of oxide-based electronic device applications. Here, we systematically studied the physical properties of LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices and compared them with the LaLa_1-xCa_xMnO₃ thin films and bulk compounds. The LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices exhibit significant enhancement of Curie temperature (T_C) beyond the corresponding thin films and bulks. Based on these results, we constructed an extended phase diagram of LaLa_1-xCa_xMnO₃ under interfacial engineering. We considered the interfacial charge transfer and structural proximity effects as the origin of the interface-induced high T_C. The structural characterizations revealed a pronounced increase of B-O-B bond angle, which could be the main driving force for the high T_C in the superlattices. Our work inspires a deeper understanding of the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.     

Emergence of the sub‐THz central peak at phase transitions in artificial BaTiO3/(Ba,Sr)TiO3 superlattices

A prominent central peak in the sub‐THz frequency range was observed in the Raman spectra of BaTiO₃/(Ba,Sr)TiO₃ (BT/BST) superlattice grown on (001)MgO substrate. Both soft and central mode show an anomaly around 200 K and 280 K, which can be correlated with orthorhombic to monoclinic phase transition of BST and BT, respectively. The observed temperature dependence of the central mode enabled us to explain rather broad temperature dependence of the dielectric permittivity previously observed in BT/BST superlattices. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Natural Soft/Rigid Superlattices as Anodes for High-Performance Lithium-Ion Batteries

Volume expansion and poor conductivity are two major obstacles that hinder the pursuit of the lithium-ion batteries with long cycling life and high power density. Herein, we highlight a misfit compound PbNbS₃ with a soft/rigid superlattice structure, confirmed by scanning tunneling microscopy and electrochemical characterization, as a promising anode material for high performance lithium-ion batteries with optimized capacity, stability, and conductivity. The soft PbS sublayers primarily react with lithium, endowing capacity and preventing decomposition of the superlattice structure, while the rigid NbS₂ sublayers support the skeleton and enhance the migration of electrons and lithium ions, as a result leading to a specific capacity of 710 mAh g⁻¹ at 100 mA g⁻¹, which is 1.6 times of NbS₂ and 3.9 times of PbS. Our finding reveals the competitive strategy of soft/rigid structure in lithium-ion batteries and broadens the horizons of single-phase anode material design.