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     

不同价态Mn掺杂InN电子结构、磁学和光学性质的第一性原理研究

采用密度泛函理论体系下的广义梯度近似GGA+U平面波超软赝势方法,在构建了纤锌矿结构的InN超胞及三种不同有序占位Mn~(2+),Mn~(3+)价态分别掺杂InN超胞模型,并进行几何优化的基础上,计算了掺杂前后体系的电子结构、能量以及光学性质.计算结果表明:Mn掺杂后体系总能量和形成能降低,稳定性增加,并在费米能级附近引入自旋极化杂质带,体系具有明显的自旋极化现象.掺杂不同价态的Mn元素对体系电子结构和磁学性质产生了不同的影响.电子结构和磁性分析表明掺杂体系的磁性来源于p-d交换机制和双交换机制的共同作用,Mn~(3+)价态掺杂有利于掺杂体系的居里温度达到室温以上.与未掺杂InN相比,不同价态Mn元素掺杂后体系的静态介电函数显著增大,掺杂体系介电函数虚部和吸收光谱在低能区域出现了较强的新峰,分析认为这些新峰主要来自与费米能级附近自旋极化杂质带相关的跃迁.     

Mg2Si:Fe 电子结构及磁性的理论研究

采用基于密度泛函理论的第一性原理赝势平面波方法研究了Mg_2Si:Fe体系的电子能带结构、态密度和磁性.结果表明:掺入的Fe原子优先占据晶格中的空隙位,也可能代替晶格中的Mg位.从能带结构和态密度可以看出,当Fe原子位于晶格中空隙位时,系统显示出金属性;当Fe占据Mg位置时,对于自旋向上电子态,体系有一带隙存在,系统呈现明显的半导体特性;对于自旋向下电子态,Fe的替位掺杂在该体系内引入新的杂质能级,杂质能级与导带价带分离,且100%自旋极化.两种位置的杂质,上自旋电子和下自旋电子的态密度均明显不对称,诱导出铁磁性,且铁磁性主要由于Fe的3d态电子诱导产生.Fe位于空隙位时,Fe原子的磁矩为1.69μB;Fe占据Mg位时,Fe原子的磁矩为1.38μB,说明原子磁矩与其所占位置和配位情况有关.     

碳和氧掺杂紫磷烯作为双极磁性半导体材料的理论预测

紫磷烯是一种结构稳定且具有优异光电特性的新型二维材料,研究掺杂效应有助于理解其物理本质,对进一步开发纳米电子器件具有重要意义.本文采用基于密度泛函理论的第一性原理方法,研究了非金属元素B,C,N,O掺杂单层紫磷烯的电磁性质.计算结果表明,B和N掺杂之后没有产生磁性,体系依旧表现为非磁性半导体;而C和O掺杂导致体系发生自旋劈裂,紫磷烯由非磁性半导体转变成为双极磁性半导体,其自旋密度主要分布在磷原子和间隙区域内而非杂原子上.电场调控氧掺杂紫磷烯可使其载流子的自旋极化方向发生反转,当施加一定大小的正向或反向的静电场时,能带色散程度变强,氧掺杂紫磷烯转变成100%自旋极化向下或向上的单自旋半金属磁体.基于氧掺杂紫磷烯材料设计的场效应自旋滤通器可利用改变门电压方向的方法实现电流自旋极化方向的反转,表明氧掺杂紫磷烯有望成为二维自旋场效应晶体管、双极磁性自旋电子学器件、双通道场效应自旋滤通器以及场效应自旋阀的理想候选材料.     

SiC单层内Co空间选位对自旋组态的调控（英文）

基于第一性原理的密度泛函理论,对SiC单层不同位置掺杂Co进行了能带结构、电子态密度、净自旋密度和自旋纹理等计算,结果表明不同位置的掺杂引起不同特征的自旋积累及单层的电子结构特性．由于Co的不同选位掺杂而产生一些新奇现象,如扭曲的Co-C键在掺杂SiC内激发了自旋流而诱导了自旋重新分布,不同选位的Co原子通过调整内磁场改变了小极化子内巡游电子的定域属性,增加了Dirac点附近磁振子的色散强度等．这些研究结果为得到一个人工调控量子自旋电路和选频自旋波器件内自旋谷电子提供了理想平台．     

原子替位掺杂对单层Janus WSeTe电子结构的影响

基于第一性原理计算系统地研究了氮族、卤族和3d过渡金属元素(Ti, V, Cr, Mn, Fe, Co)替位掺杂对单层Janus过渡金属硫族化合物WSeTe电子结构的影响.通过对能带结构、电荷转移以及磁性的分析,发现氮(卤)族原子替位掺杂单层WSeTe会发生本征半导体-p (n)型半导体的转变, Ti, V原子替位掺杂单层WSeTe会发生半导体-金属的转变.由于电荷转移以及氮族原子掺杂时价带顶的能带杂化现象,卤族和氮族非金属元素掺杂时价带顶G点附近的Rashba自旋劈裂强度在同一主族随着掺杂原子原子序数的增大而增大. 3d过渡金属元素掺杂会产生能谷极化和磁性,其中Cr, Mn原子替位掺杂会产生高于100 meV的能谷极化,并且Cr,Mn, Fe元素掺杂在禁带中引入了电子自旋完全极化的杂质能级.研究结果对系统地理解单层WSeTe掺杂模型的性质具有重要意义,可以为基于单层WSeTe的电子器件设计提供理论参考.     

C-Nb共掺杂SnO_2电子结构的第一性原理研究

本文利用第一性原理研究了C-Nb共掺杂的SnO_2稳定性、能带结构与态密度,从自旋向上和自旋向下的能带结构以及态密度分析了掺杂体系磁性产生的机理.研究结果表明,C-Nb共掺杂SnO_2体系的稳定性强于C,Nb单掺杂SnO_2体系;C,Nb单掺杂、C-Nb共掺杂的SnO_2体系的总磁矩分别为0μB、0.922μB、1.0μB;Nb掺杂SnO_2体系产生磁性在于Nb的d轨道引入,C-Nb共掺杂SnO_2体系产生磁性在于Nb的s轨道和C的p轨道相互作用.     

硅团簇自旋电子器件的理论研究

利用过渡金属掺杂的硅基团簇, 构建了一种自旋分子结; 并利用第一性原理方法, 对其电子自旋极化输运性质进行了研究. 计算表明, 通过过渡金属掺杂可以有效地产生自旋极化电流, 磁性金属Fe和非磁性金属Cr和Mn掺杂的体系呈现出较明显的自旋极化透射现象, 但分子结的自旋极化输运能力与团簇孤立状态下的磁矩无一致性.从Sc到Ni的掺杂, 体系的自旋极化透射能力先增大后迅速减小, 在Fe掺杂的Si₁₂团簇中出现最大值. 关键词： 硅团簇 自旋极化输运 密度泛函理论 非平衡格林函数     

Sc掺杂SnO2稀磁半导体的第一性原理研究

采用第一性原理方法,对Sc掺杂SnO2以及含有O空位的Sc掺杂SnO2的电子结构和磁学性质进行了计算。结果表明SnO2晶格中存在两种本征磁性来源,分别为Sc掺杂诱导的未配对O-2p态电子的自旋极化和O空位诱导的未配对Sn-5p态电子的自旋极化。由于两种未配对的弱束缚电子分别由电离施主和受主诱导产生,因此二者之间存在电荷补偿效应,在特定配比下能够使SnO2晶格出现磁性猝灭。     

SiC单层内Co空间选位对自旋组态的调控

基于第一性原理的密度泛函理论,对SiC单层不同位置掺杂Co进行了能带结构、电子态密度、净自旋密度和自旋纹理等计算,结果表明不同位置的掺杂引起不同特征的自旋积累及单层的电子结构特性。由于Co的不同选位掺杂而产生一些新奇现象,如扭曲的Co-C键在掺杂SiC内激发了自旋流而诱导了自旋重新分布,不同选位的Co原子通过调整内磁场改变了小极化子内巡游电子的定域属性,增加了Dirac点附近磁振子的色散强度等。这些研究结果为得到一个人工调控量子自旋电路和选频自旋波器件内自旋谷电子提供了理想平台。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

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.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.