钙钛矿锰氧化物低维纳米结构研究进展

钙钛矿结构锰氧化物由于同时存在电荷、自旋、轨道、晶格等多种自由度, 它们之间很强的相互作用和相互竞争导致了一系列新颖的物理现象, 如庞磁电阻效应、巨磁熵效应、绝缘体-金属转变、电子相分离、电荷/轨道有序等现象, 使其成为凝聚态物理学研究的热点。随着微电子器件日趋集成化和微型化, 其特征尺寸越来越小, 目前基于钙钛矿结构锰氧化物微电子器件的特征尺寸已经进入纳米尺度。在纳米尺度钙钛矿结构锰氧化物具有显著的尺寸效应, 表现出与薄膜及块材不同的电、磁输运特性, 在新一代微电子器件领域具有重要的应用价值。近年来人们在钙钛矿锰氧化物低维纳米结构制备、电磁输运特性测量、微结构表征及理论模拟方面, 都取得了较大的研究进展, 本文对此进行了评述。首先, 概述了钙钛矿锰氧化物低维纳米结构的微结构研究进展; 介绍了钙钛矿锰氧化物低维纳米结构的电子相分离及电荷有序现象; 评述了其电磁输运特性的纳米尺度表征; 讨论了钙钛矿锰氧化物低维纳米结构在自旋电子学、磁随机存储器和传感器方面的应用进展。最后指出了未来钙钛矿锰氧化物低维纳米结构研究需要重点解决的一些问题。     

Kosterlitz-Thouless Transition in Finite-Size BEC Systems

The Kosterlitz-Thouless (KT) transition in two-dimensional BEC systems is calculated taking into account the fact that in experiments these are finite-size systems. The outer boundaries of the condensate effectively act as hard walls, and this has a polarizing effect on the vortex pairs of the KT transition, causing the superfluid fraction to become strongly anisotropic. The decreased pair energy near the walls results in a strongly enhanced vortex density near the boundaries. Since the pair density can now be directly measured, we extend here our previous calculations to include the vortex density as a function of the distance from the boundaries. Possible experiments using sound propagation in the gases are proposed to probe the anisotropic properties of the superfluid density, including an unusual sharp dip in the superfluid density that is predicted to occur down the middle of a long superfluid strip.      

Low-dimensional chaos and wave turbulence in plasmas

We investigated drift-wave turbulence in the plasma edge of a small tokamak by considering solutions of the Hasegawa-Mima equation involving three interacting modes in Fourier space. The resulting low-dimensional dynamics presented periodic as well as chaotic evolution of the Fourier-mode amplitudes, and we performed the control of chaotic behaviour through the application of a fourth resonant wave of small amplitude.     

低维度薄膜传热机理与表征

低维度薄膜传热学是建立在微小器件研究基础上的纳米学科分支之一。综述了细微尺度传热的主要方式,着重介绍了对流换热、热传导、热辐射、相变传热、微重力传热;详细阐述了导热系数的表征方式,包括温度传感器法、光声法、显微拉曼散射法、激光泵浦法、温度探测扫描电镜法、动态热线法。最后,分析与评述了国内外研发应用现状。     

Vortex Lattice Dynamics in a Dilute Gas BEC

We study the dynamics of large vortex lattices in a dilute gas Bose–Einstein condensate. Rapidly rotating condensates are created that contain vortex lattices with up to 300 vortices. The condensates are held in a parabolic trapping potential, and rotation rates exceeding 99% of the radial trapping frequency are achieved. By locally suppressing the density while maintaining the phase singularities, we create vortex aggregates. To illustrate the underlying Coriolis force driven dynamics, oscillation frequencies of the vortex aggregate area are measured. A related technique also enables us to excite and directly image Tkachenko modes in a vortex lattice. These modes provide evidence for the shear strength that a vortex lattice in a superfluid can support.     

氧化锌低维结构材料场发射性能研究进展

ZnO具有负的电子亲和势、丰富的结构和形貌可设计性以及良好的机械和化学稳定性等优点,是一种最有前途的阴极电子发射材料.结合作者实验室的工作,综述了近年来迅速发展的ZnO低维结构的场发射特性以及制备方法、形貌结构、排列及密度、表面吸附、掺杂和热处理等因素对其场发射性能的影响,介绍了四针状氧化锌晶须(T-ZnOw)作为场发射阴极材料的优势.     

激光辅助制备低维纳米材料的研究进展

介绍了激光辅助制备低维纳米材料的几种方法,论述了激光法制备低维纳米材料的国内外研究进展.同时提出了该方法研究的重点并对其今后的发展趋势进行了展望.     

Low-dimensional SiC nanostructures: Fabrication, luminescence, and electrical properties

Nanostructured silicon carbide has unique properties that make it useful in microelectronics, optoelectronics, and biomedical engineering. In this paper, the fabrication methods as well as optical and electrical characteristics of silicon carbide nanocrystals, nanowires, nanotubes, and nanosized films are reviewed. Silicon carbide nanocrystals are generally produced using two techniques, electrochemical etching of bulk materials to form porous SiC or embedding SiC crystallites in a matrix such as Si. Luminescence from SiC crystallites prepared by these two methods is generally believed to stem from surface or defect states. Stable colloidal 3C-SiC nanocrystals which exhibit intense visible photoluminescence arising from the quantum confinement effects have recently be produced. The field electron emission and photoluminescence characteristics of silicon carbide nanostructures as well as theoretical studies of the structural and electronic properties of the materials are described.     

Spin Superfluidity, Coherent Spin Precession, and Magnon BEC

Spin superfluidity, coherent spin precession, and magnon BEC are intensively investigated theoretically and experimentally nowadays. Meanwhile, clear definition and differentiation between these related phenomena is needed. It is argued that spin stiffness, which leads to existence of coherent spin precession and dissipationless spin supercurrents, is a necessary but not sufficient condition for spin superfluidity. The latter is defined as a possibility of spin transport on macroscopical distances with sufficiently large spin supercurrents. This possibility is realized at special topology of the magnetic-order-parameter space, such as, e.g., that in easy-plane antiferromagnets. It is argued that an arbitrarily chosen formal criterion for the existence of magnon BEC has no connection with conditions for observation of macroscopic dissipationless spin transport.     

Topological Phase Imprinting in BEC in Presence of Gravitational Field

No Heading We report behavior of BEC during topological phase imprinting under the influence of a gravitational field. When the gravitational field is perpendicular to the vortex line as in the experiments, the system is no longer axisymmetric and the vortex is not always stable. We solved the time-dependent Gross-Pitaevskii equation numerically and found that a stable vortex is produced by fine-tuning the field reverse time.PACS numbers: 03.75.Mn, 67.57.Fg.     

Fluctuations and BEC at the Free Surface of 4He

There is some theoretical evidence that full Bose–Einstein condensation (BEC) is achieved at the free surface of superfluid ⁴ He where the density is small. We present new variational results of the BEC and of the density fluctuation spectrum in the surface region of a slab of ⁴ He. We use both a standard wave function (wf) with bulk correlations and one body shape terms and a novel shadow wave function with a glue term (G-SWF). This last one describes the self- binding of ⁴ He only via interparticle correlations. In both cases we find that BEC increases from the bulk like value well inside the slab to a much larger value in the surface region but a striking different behavior is found in the low density region. With a standard wf the BEC reaches essentially 100% in the surface region. With the G-SWF the condensate only increases up to about 51%. Further out of the surface the condensate decreases and correspondingly there is an enhanced population of small momentum states. This different behavior is correlated with the presence in the G-SWF of enhanced density fluctuations in the surface region due to the zero point motion of ripplons. This enhancement is absent in the case of a standard wf.     

BEC and Dimensional Crossover in a Boson Gas Within Multi-slabs

For an ideal Bose-gas within a multi-slabs periodic structure, we report a dimensional crossover and discuss whether a BEC transition at T _c ≠0 disappears or not. The multi-slabs structure is generated via a Kronig-Penney potential perpendicular to the slabs of width a and separated by a distance b. The ability of the particles to jump between adjacent slabs is determined by the hight V ₀ and width b of the potential barrier. Contrary to what happens in the boson gas inside a zero-width multilayers case, where the critical temperature diminishes and goes up again as a function of the wall separation, here the T _c decreases continuously as the potential barrier height and the cell size a+b increase. The specific heat shows a crossover from 3D to 2D when the height of the potential or the barrier width increase, in addition to the well known peak related to the Bose-Einstein condensation.     

Functional Integral Approach Study of Spatial Distribution for an Attractive BEC

We report a study of Bose–Einstein condensation (BEC) in a system of bosons with a negative scattering length subjected to a harmonic-oscillator confining potential. The spatial and momentum distributions for dilute Bose gases are obtained exactly in the forward-scattering-dominated (FSD) model by the functional integral approach. We demonstrate the possibility of BEC for a Bose system with an attractive interaction before its collapse. Numerical calculations are also presented Brigham Young University emeritus     

生物分子模板法制备低维金属纳米材料研究进展(I)

概述了脂类、蛋白质、DNA等生物分子模板表面化学沉积制备低维金属纳米材料的最新研究进展.脂类、蛋白质和DNA分子可自组装形成不同的纳米结构,如纳米管和纳米线等.这些不同的纳米结构可作为模板,化学沉积制备不同的低维金属纳米材料.金属纳米管具有高强度、导电、导热、磁性,在电活性复合材料,药物和海洋防污剂的可控缓释等领域有重要的应用;金属一维纳米线具有特殊的量子效应,可用于纳米器件和纳米电路的开发.该研究可以帮助人们将生物学知识转变为材料学知识,该方法是分子自组装纳米材料和纳米结构走向工程应用的重要途径之一.     

Microwaves Interaction Peculiarities with the Ideal Gas of Alkali Atoms in BEC State

We represent a theoretical approach that is based on approximate formulation of the second quantization method at low temperatures. It allows to study a system consisting of alkali atoms response to the perturbation by the external electromagnetic field. We show that such approach is convenient to study the propagation properties of microwave signal, tuned up to the transition between hyperfine ground state levels of alkali atoms that are considered in Bose-Einstein condensation (BEC) state. The phenomenon is studied in case of hyperfine levels Zeeman splitting. The dependence of pulse slowing on the system parameters is found. It is shown that a slowed signal can propagate in such system with rather small energy loss. A possibility to observe ultra-slow microwaves in the condensed gas of cesium atoms is discussed.      

低维钙钛矿:兼具高效率和稳定性的新型太阳能电池光吸收层候选材料

使用有机无机杂化钙钛矿材料作为光吸收层的钙钛矿太阳能电池自进入人们的视野以来,其制备工艺和器件结构不断得到优化,短短几年内效率取得了非常可观的增长。与此同时,这种基于三维钙钛矿材料的电池的缺点也越来越突出,尤其是材料的不稳定性,严重阻碍了其发展。低维钙钛矿材料具有有机胺层与无机层(金属卤化物钙钛矿晶体)之间相互交替的低维(层状)结构,其中被有机胺隔开的独立钙钛矿层中八面体的层数n越小,钙钛矿越接近二维结构。相比传统三维钙钛矿结构,低维钙钛矿材料应用于光伏器件具有两大优势:(1)耐湿性、光热稳定性大大增强;(2)可以通过改变n和插入的有机胺的种类来实现光学及电学性质的可调性。然而,低维钙钛矿具有较大的光学带隙,有机胺的引入降低了载流子迁移率,导致低维钙钛矿电池的效率明显低于三维钙钛矿电池。因此,近三年来除研究钙钛矿层数对材料性质和器件性能的影响外,研究者们主要从选择合适的有机胺和优化薄膜制备工艺方面不断尝试,并取得了丰硕的成果,在充分发挥低维钙钛矿稳定性优势的同时大幅提升了器件效率。目前,低维钙钛矿太阳能电池的光电转换效率已由2014年的4.37%跃升至13.7%。在较高效率的低维钙钛矿太阳能电池中已取得成功应用的有机胺类包括苯乙胺(PEA)、正丁胺(n-BA)、异丁胺(iso-BA)、聚乙烯亚胺(PEI)等。其中PEA应用得最早;n-BA是运用在目前为止最高效的低维钙钛矿电池中的有机胺;而PEI插层形成的低维钙钛矿拥有相对更小的光学带隙和更高的耐湿性,但载流子的传输会受到更大的限制。低维钙钛矿薄膜的制备起初主要采用简单的一步旋涂法,但此法所得的低维钙钛矿平行于基底生长,器件效率很低。近两年的研究工作将基底预热、浸泡、反溶剂滴加等手段引入到钙钛矿旋涂工艺中,实现了低维钙钛矿优先垂直基底生长,为突破低效率瓶颈提供了可能。此外,以三维钙钛矿为基础,以有机胺为添加剂,制得的二维和三维混合的钙钛矿结构,也可以实现器件效率和稳定性的双提升。本文归纳了低维钙钛矿光伏器件的研究进展,分别对低维钙钛矿的分子结构、插入的有机胺的选择、钙钛矿薄膜的制备方法等进行介绍,分析了低维钙钛矿太阳能电池面临的问题并展望其前景,以期为制备稳定和环境友好的新型钙钛矿太阳能电池提供参考。     

沉淀法制备低维纳米AlO(OH)的工艺条件研究

在不同工艺条件下采用沉淀法制备了低维纳米AlO(OH),并对样品进行了XRD、FT-IR及SEM表征分析。结果表明,pH值较低和较高时,样品的结晶度均较低。反应温度为75℃和95℃时,样品的结晶度均较好,无杂质。当pH值为9.0、反应温度为85℃时,样品中存在大量缔合-OH,AlO(OH)结晶度较好,且有少量Al(OH)3晶体存在。由于SDBS在Al(OH)3颗粒表面形成完整胶束,且有空间位阻效应,此时形成的样品为5nm厚、长宽可达100~300nm的边缘卷曲的蓬松片状物,形貌规则、尺寸均匀、分散性较好。