CoTiSb基体中Ni元素诱导的单自旋通道研究

沿半Heusler结构CoTiSb合金的[001]晶体学方向, 利用Ni元素连续替换一条原子链上的Ti, Sb原子, 在半导体性CoTiSb基体中设计了一系列均匀分布的Ni基单原子链阵列. 采用第一性原理方法, 研究了Ni基单原子链的电子结构和磁性质, 发现Ni-Sb单原子链具有高度自旋极化率和空穴导电特性, Ni-Ti及Ni-Ni单原子链具有100%的自旋极化率, 并且在CoTiSb基体中形成了以这种Ni基单原子链为中心的、尺寸非常小的单自旋纳米柱通道.     

第一性原理研究Hg2CuTi型Heusler合金Ti2FeB的磁性和半金属性

基于密度泛函理论的GGA计算,我们具体研究了Hg2 CuTi型Heusler合金Ti2 FeB的电子结构和磁性质,结果发现Ti2FeB合金在其费米面处存在100％的自旋极化,并在5.1～6.2(A)晶格范围内被保留.Ti2FeB具有大约0.5 eV的半金属带隙和1μB的原胞总磁矩,是一个稳定的半金属铁磁体.此外,我们的研究也表明RKKY型间接交换和d电子杂化在决定合金磁性质中起决定性作用.     

第一性原理研究Hg2CuTi型Heusler合金Ti2FeB的磁性和半金属性

基于密度泛函理论的GGA计算,我们具体研究了Hg2CuTi型Heusler合金Ti2FeB的电子结构和磁性质,结果发现Ti2FeB合金在其费米面处存在100%的自旋极化,并在5.1-6.2Å;晶格范围内被保留。Ti2FeB具有大约0.5eV的半金属带隙和1μB的原胞总磁矩,是一个稳定的半金属铁磁体。此外,我们的研究也表明RKKY型间接交换和d电子杂化在决定合金磁性质中起决定性作用。     

电磁波辐照下量子线的电子自旋极化输运性质

理论上研究Rashba自旋-轨道相互作(SOI)量子线在外电磁波辐照下的电子自旋极化输运性质.在自由电子模型下利用散射矩阵方法,发现当Rashba SOI较弱时,自旋极化率与外电磁场频率和电子入射能量无关,而当Rashba SOI较强时,自旋极化率则强烈依赖于外场频率和电子入射能量,其物理根源是Rashba SOI使子带混合引起的.此外,当电子的入射能量增加到打开另一通道阈值时,电子的透射率出现一个反常的台阶结构,这来源于电子与光子的非弹性散射而使电子在子带间的跃迁. 关键词： 量子线 电磁波 自旋极化输运 散射矩阵     

Inverse-Heusler合金Ti_2NiAl/GaAs隧道异质结的自旋极化和电磁特性

基于密度泛函理论的第一性原理计算,研究了Hg_2CoTi型Inverse-Heusler合金Ti_2NiAl和GaAs异质界面上的原子相互作用、磁性、态密度和自旋极化.结果表明,在所研究的12种异质结构(包括顶位和桥位连接)中,界面态的存在不同程度地破坏了合金的半金属性,导致不超过75%的自旋极化率发生;仅当Heusler合金TiAl端面的Al原子或者TiNi端面的Ti原子位于As原子的顶位时,合金异质结可以保留着约72%的自旋极化率,根据Julliere模型,能最大实现68.9%的自旋电子迁移率,有望在隧穿磁阻中实际应用.     

影响磁性pn结自旋极化输运特性的因素

利用漂移扩散理论研究了磁性pn结中自旋的输运特性.探讨了外加电压、平衡自旋极化率、外加自旋注入和自旋寿命对磁性pn结电流密度和电阻的影响,讨论了磁性pn结自旋伏特效应与pn结宽度的关系.发现平衡自旋极化率使得不同自旋方向电子具有不同的势垒高度从而能有效调制电流;而外加自旋注入则为磁性pn结提供了非平衡自旋极化电子从而达到对电流的调制作用,同时发现自旋伏特电流随准中性p区宽度减小而增大. 关键词： 磁性pn结 自旋极化率 自旋寿命 自旋伏特效应     

自旋极化电子从铁磁金属注入半导体时自旋极化的计算

对自旋极化电子从铁磁金属通过绝缘层薄膜注入半导体时的自旋极化率与绝缘层厚度以及所加偏压的关系等作了计算.所得结果与最新实验结果相符，并发现偏压适中、绝缘层较厚时 有较大的电流自旋极化率，偏压很小时电流自旋极化率几乎为零. 关键词： 自旋极化电子注入 Slonczewski模型 隧道磁电阻 非零偏压     

对称双势垒量子阱中自旋极化输运的时间特性

电子的隧穿时间是描述量子器件动态工作范围的重要指标. 本文考虑k³ Dresselhaus 自旋轨道耦合效应对系统哈密顿量的修正, 结合转移矩阵方法和龙格-库塔法来解含时薛定谔方程, 进而讨论了电子在非磁半导体对称双势垒结构中的透射系数及隧穿寿命等问题. 研究结果发现:由于k³ Dresselhaus 自旋轨道耦合效应使自旋简并消除, 并在时间域内得到了表达, 导致自旋向上和自旋向下电子的透射峰发生了自旋劈裂; 不同自旋取向的电子构建时间和隧穿寿命不同, 这是导致自旋极化的原因之一; 电子的自旋极化在时间上趋于稳定. 关键词： 自旋极化输运 透射系数 隧穿寿命 自旋极化率     

自旋相关的电子隧穿和自旋极化

研究了电子的自旋相关的隧穿和极化。在外加磁场的作用下，自旋向上的电子与自旋向下的电子具有不同的隧穿系数。当电子的自旋方向与磁场方向相反时，其隧穿概率受到磁场的抑制而变小；反之，当两平行时，电子的了隧穿系数增大。这种差异可以用本中定义的自旋极化率来表示。本对不同磁场下的自旋极化率进行了计算，结果也表明当电子的动能较小，这种自旋极化的效应越显。     

N掺杂钛酸盐的电子结构和磁性研究

基于密度泛函理论,从头计算了N掺杂立方结构钛酸盐的电子结构和磁学性质.结果表明,N掺杂钛酸盐在自旋极化状态下的总能量比自旋非极化状态下的总能量小,说明N掺杂钛酸盐的基态具有铁磁性.从态密度和自旋密度分析可知,其磁性源于掺杂的N 2p电子和价带顶O 2p电子间的p-p耦合作用. 关键词： 钛酸盐 电子结构 磁学性质     

Self-organization of five species in a cyclic competition game

Cyclic competition game models, particularly the “rock–paper–scissors” model, play important roles in exploring the problem of multi-species coexistence in spatially ecological systems. We propose an extended “rock–paper–scissors” game to model cyclic interactions among five species, and find that two of the five can coexistent when biodiversity disappears, which is different from the “rock–paper–scissors” game. As the number of fingers is five, we named the new model the “fingers” game, where the thumb, forefinger, middle finger, ring finger, and little finger cyclically dominate their subsequent species and are dominated by their former species. We investigate the “fingers” model in two ways: direct simulations and nonlinear partial differential equations. An important finding is that the number of species in a cyclic competition game has an influence on the emergence of biodiversity. To be specific, the “rock–paper–scissors” model is in favor of maintaining biodiversity in comparison with the “fingers” model when the variables (population size, reproduction rate, selection rate, and migration rate) are the same. It is also shown that the mobility and reproduction rate can promote or jeopardize biodiversity.     

Contrastive analysis on tone between Putonghua and Taiwan Mandarin

The differences of the pitch and duration of Chinese syllables between Putonghua （PTH） and Taiwan Mandarin （TM） were studied. The speech materials to be used are not only isolated syllables, but also sentences. The results reveal that： For the isolated syllables, T1 and T2 in TM are influenced by Minnan dialect, therefore their pitch are lower than those in PTH. T3 is fall-rise in PTH, while it is fall in TM. Moreover, the syllable duration sequence for different tone is T3〉T2〉T1〉T4 in PTH, while it is T1〉T2〉T3〉T4 in TM. For the syllables in sentences, T2 is mid-rise in PTH, while it is mid-level in TM. And the T3 is longer than T4 but shorter than T1 or T2 in PTH, while it is the shortest in TM. Furthermore the effects of prosodic phrase boundary on duration for different tones are almost the same in PTH, but the lengthening part of T1 or T2 is longer than that of T3 or T4 in TM.     

Experimental study on downstep in Chinese intonation

Downstep in pitch contour of Chinese Putonghua is examined using subtly designed sentences by controlling tone combinations. The results show both automatic and nonautomatic downstep phenomena exist in Chinese. In non-automatic downstep, low tones compress downwards the pitch range of the following syllables. and the main influence of downstep is on topline. Low tone not only lower the topline behind it, but also raise the high tones before it, the effects are compatible with each other. In automatic downstep, the topline of pitch contour in intonational phrase is presented as a linear downtrend, but it differs among speakers due to the effect of personal stress practice. In comparison with downstep phenomenon in other tone or non-tone languages, the downstep ratio in Chinese is not constant, and the domain of downstep is not limited within the adjacent tones.     

Abelian-Higgs phase of SU(2) QCD and glueball energy

It is shown that SU(2) QCD admits an dual Abelian-Higgs phase, with a Higgs vacuum of a type-Ⅱ superconductor. This is done by using a connection decomposition for the gluon field and the random-direction approximation. Using a bag picture with soft wall, we presented a calculational procedure for the glueball energy based on the recent proof for wall-vortices [Nucl. Phys. B 741(2006)1].     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Signal generation in an isotropic medium in scanning electron acoustic microscope

Based on the research in Ref. [5][Materials Science and Engineering, 1989; A122： 57 63], an improved model of heat source is set up, the different modes of Lamb wave in an isotropic sample generated by a chopped electron beam at frequency f are obtained with integral transform and normal function expansion method, and the output signal of PZT coupled at the back surface of the sample is found out. The generation mechanism of SEAM （Scanning Electron Acoustic Microscopy） signal is discussed. It shows that the SEAM is a near field imaging technique with high spatial resolution and its best lateral spatial resolution is about 2√2α （α is the radius of the focused electron beam）. Some of experimental results of SEAM images are presented in the paper and it shows that the spatial resolution of SEAM is better than 0.5 μm and smaller than the thermal diffusion length of the sample. Therefore the character of near field imaging in SEAM is also proved experimentally.     

High efficiency filtering using two-dimensional photonic crystal coupled cavity structures

Transmission spectra of coupled cavity structures (CCSs) in two-dimensional (2D) photonic crystals (PCs) are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain (FDTD) method, and the results show that within a very short coupling distance of about 3λ, where λ is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.