n=2 Ruddlesden-Popper Sr_3B_2Se_7(B=Zr,Hf)非常规铁电性的第一性原理研究

近年来,层状钙钛矿材料中存在的非常规铁电性为新型铁电体设计提供了新的途径.基于第一性原理,本文系统研究了具有Ruddlesden-Popper (RP)(n=2)结构的Sr_3B_2Se_7 (B=Zr, Hf)化合物的基态结构、电子结构和非常规铁电性.研究表明, Sr_3B_2Se_7 (B=Zr, Hf)基态均为具有A2_1am极性相的直接带隙半导体;其非常规铁电性来源于BSe_6八面体的两种旋转模式的耦合.而且,因具有较强的铁电极化值与可见光吸收带隙, Sr_3B_2Se_7 (B=Zr, Hf)有望成为新一代铁电光伏材料.     

SiP化合物结构和电子特性的第一性原理研究

应用第一性原理方法研究了SiP化合物的结构和电子特性,并且将研究推广到其他第四族元素磷化物(IV-P).在研究的各种结构中,SiP单斜晶体结构是能量最低、最稳定的结构.SiP的体弹性模量比CN和CP化合物以及相对应的第三族元素氮化物和磷化物要小.SiP不同的结构间能发生相变,其单斜晶体结构(monoclinic)在压强为6.2 GPa, 15.0 GPa, 19.3 GPa, 20.0 GPa 和 10.3 GPa时分别转变成GeP型结构、Rhom.型结构、β-InS型结构、 CsCl型结构和NaCl型结构.能带计算结果显示SiP单斜晶体结构(monoclinic)和GaSe型结构是间接带隙分别为1.123 eV 和 0.123 eV的半导体,SiP其他结构则显示出金属特性.其他化学比为1:1的第四族元素磷化物(IV-P)具有相同的性质.     

用MOS方法研究四元混晶In_(0.75)Ga_(0.25)As_(0.58)P_(0.42)中的深能级中心

采用MOS方法,在液相外延生长的n型In_(0.75)Ga_(0.25)As_(0.58)P_(0.42)混晶中,我们测出了两个位于导带下面0.20eV和0.48eV处的电子陷阱,对电子的俘获截面分别为1.4×10~(-16)cm~2和3.8×10~(-12)cm~2。 关于采用MOS方式研究具有多层结构的化合物半导体材料的DLITS问题,本文也进行了讨论。     

同时具有CuO_2层和Fe_2Se_2O层的新型结构Pr_4CuO_7Fe_2Se_2的设计与电子结构研究

铜氧化物高温超导体和铁基高温超导体的发现,打破了基于电声耦合机理的BCS所预测的超导转变温度极限,使得人们对于超导材料的研究进入新的阶段.两者具有相似的层状结构,在掺杂引入载流子的情况下能够产生超导现象.本文报道基于已有的铜氧化物超导体Pr2CuO_4和具有类超导体结构的Pr2O_3Fe_2Se2系列化合物,设计出一系列超导候选材料Pr_4CuO_7Fe_2Se_2,并对其电子结构进行了基于密度泛函理论的第一性原理计算研究.计算的能带结构和态密度表明,Pr_4CuO_7Fe_2Se_2整体表现出弱导电性,与已有超导体类似;具有明显的各项异性,同样为层状结构,Fe_2Se_2O层和CuO_2层均表现出导电性,被Pr2O_2层分离开,其中费米能级附近Fe_2Se_2O层对态密度的贡献较大,Pr2O_2层几乎无导电性,z轴方向则表现出绝缘体特性.新设计结构的整体表现如同Pr2CuO_4和Pr2O_3Fe_2Se2的结合,但是CuO_2层和Fe_2Se_2O层的活跃能级区域又有所不同,这就使得同一环境下,单一物质中,对比研究两种层状结构的物理表现成为一种可能.     

二维GaTe/Bi_2Se_3异质结:一类有潜力的Z型光解水催化剂（英文）

本文基于第一性原理方法,计算了二维GaTe/Bi_2Se_3异质结的电子结构、界面电荷转移、静电势分布、吸收光谱及光催化性质.计算结果表明异质结是一个小能隙的准直接半导体,能有效捕获太阳光.由于相对较强的界面内建极化电场和带边轻微弯曲,导致异质结中的光生电子和空穴分别有效分离在GaTe单层和Bi_2Se_3薄片上,可用于析氢和产氧.这些理论计算结果意味着二维GaTe/Bi_2Se_3异质结是一类有潜力的Z型太阳能全解水催化剂.     

Cu2ZnSnS4类四元硫族半导体的理论研究——以二元、三元、四元半导体的演化为思路

在过去60多年中,人们对半导体的研究集中在一元、二元和三元半导体方面,最近,出于寻找新型廉价、环保、高效光伏转换材料的需要,Cu2ZnSnS4类Ⅰ2-Ⅱ-Ⅳ- Ⅵ4型四元硫族半导体吸引了人们越来越多的关注,它在光催化和热电等多方面的应用也不断被发掘.然而,对于这类四元半导体的基本性质,如晶体结构和电子结构,人们知之甚少,很多研究还停留在经验阶段.文章首先简要回顾了这类半导体的由来和在应用方面的最新进展,然后详细介绍了文章作者对这类四元半导体的第一性原理计算研究工作的进展,其中包括:系统研究了这类硫族半导体在从二元向三元再向四元的演化过程中晶体结构和电子能带结构变化的规律,总结了元素成分对其影响的一般趋势,并结合实验结果分析了这类四元半导体晶格结构表征和带隙测量中易于出现的混淆;文章作者还以Cu2ZnSnS4为例,考察了这类四元化合物相对二元、三元化合物的相稳定性和本征缺陷性质.文章介绍的研究结果将为一系列Ⅰ2-Ⅱ-Ⅳ- Ⅵ4型四元半导体的深入研究提供基础.     

Quantum oscillations and nontrivial transport in(Bi_(0.92)In_(0.08))_2Se_3

Quantum phase transition in topological insulators has drawn heightened attention in condensed matter physics and future device applications.Here we report the magnetotransport properties of single crystalline(Bi_(0.92)In_(0.08))_2Se_3.The average mobility of～1000 cm~2·V~(-1)·s~(-1)is obtained from the Lorentz law at the low field(3 T)up to 50 K.The quantum oscillations rise at a field of～5 T,revealing a high mobility of～1.4×10~4cm~2·V~(-1)·s~(-1)at 2 K.The Dirac surface state is evident by the nontrivial Berry phase in the Landau–Fan diagram.The properties make the(Bi_(0.92)In_(0.08))_2Se_3a promising platform for the investigation of quantum phase transition in topological insulators.     

硫钒铜矿化合物电子结构和电致变色特性的第一原理研究

基于密度泛函的第一原理赝势平面波方法,计算晶体结构、电子结构和光学性质,研究硫钒铜矿化合物Cu₃VS₄、Cu₃NbS₄和Cu₃TaS₄的电子输运及电致变色特性,探讨作为透明半导体材料应用于太阳能电池和电致变色器件的可能性.电子结构的计算表明这类化合物是间接带隙半导体,其电子能带的导带底和价带顶分别位于布里渊区的X点和R点.价带顶的电子本征态主要来自于Cu原子的d电子轨道,而导带底电子态主要来源于VB族元素原子的d电子轨道.能带结构、电荷布居分析、电子局域化函数和光吸收及反射谱的计算表明这些硫钒铜矿化合物属于极性共价半导体,具有较高的电荷迁移率和优良的电致变色特性,可应用于高效电致变色器件.     

Sn掺杂对In_2O_3热电性能的影响

本文利用第一性原理结合半经典玻尔兹曼理论研究了Sn掺杂对In_2O_3热电特性的影响.由于一个In_2O_3原胞有80个原子,所以为了清楚表明Sn的掺杂浓度,我们将化学式表述为In_(32-x)Sn_xO_(48).形成能的计算表明Sn比较容易取代In位,且Inb位比Ind位更容易被取代.且只有x=1,形成能是负值,而x=2和3的形成能是正值.电子结构的计算表明Sn掺杂对In_2O_3的能带结构的形状影响很小,只是费米能级向导带方向移动了,基于这一点我们预测刚性带模拟In_2O_3的电子热电特性和实际Sn掺杂的应该比较接近.输运性质的计算表明在价带顶或导带底附近,电子输运性质随化学势发生明显的变化,而在价带以上导带以下的一定化学势范围内,虽然S,σ/τ和n随化学势和温度变化比较大,ZeT随化学势和温度几乎没有变化,且n型和p型掺杂下的ZeT非常接近,大小在1附近.令人兴奋的是,通过将刚性带模型计算In_2O_3电子输运性质和实验结果对比,发现当温度为1000 K,化学势为0.6512 Ry时的实验ZT=0.28和理论0.273非常接近.而此化学势远在导带底以上,说明如果选择较低的掺杂浓度,In_2O_3的输运性质有望进一步提高.     

拓扑绝缘体Bi_2Se_3中层堆垛效应的第一性原理研究

运用第一性原理方法,研究了拓扑绝缘体Bi_2Se_3块体和薄膜中的层堆垛对其结构、电子态、拓扑态和自旋劈裂的影响.发现不同的堆垛会引起Bi_2Se_3层间的相互作用,改变系统的中心对称性.块体的ABC和AAA堆垛都具有中心对称性和相似的能带结构.ABA堆垛破坏了体系的中心对称性,能带发生很大改变,并且产生了很大的能带自旋劈裂.用能带反转的方法判定体系的拓扑相,在不同堆垛的Bi_2Se_3块体中,考虑自旋轨道耦合时都发生了能带反转,因而具有不同堆垛的Bi2Se3仍是拓扑绝缘体.进一步研究了Bi_2Se_3薄膜中的堆垛效应,发现非中心对称的ABA堆垛在Bi_2Se_3薄膜中引起明显的自旋劈裂,并且提出和验证了用应变调控自旋劈裂的方法.     

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.     

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.     

Fiber Modal Index Measurements Based on Fiber Gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.     

Optical Pulse Compression Schemes That Use Nonlinear Bragg Gratings

Nonlinear optical pulse compression of picosecond pulses typically requires long lengths of optical fiber and multiple components. Periodic structures, such as fiber Bragg gratings, are highly dispersive at wavelengths outside of the photonic bandgap. This implies that such gratings can be used as very short all-fiber compressors. In this paper a number of such compression schemes are reviewed involving uniform and nonuniform fiber Bragg gratings, relying on both soliton and nonsoliton compression principles. Experimental results supporting the corresponding underlying theory are also presented. Finally, an extension of one of the compression schemes is shown, which allows the generation of adjustable high-repetition rate soliton trains.