MgB2超导体的正电子湮没研究

本文利用正电子湮没技术(PAT)对不同密度的MgB2样品分别进行了测定,结果发现正电子在MgB2中自由态的湮没时间(本征寿命τ1)和在捕获态的湮没时间(缺陷寿命τ2)比其他高温超导体都明显要高,这反映了MgB2的体电子浓度相对于其它超导体要低,可能归因于MgB2超导材料本身的晶体结构和化学组成,MgB2超导材料的正电子寿命不仅与晶体的结构缺陷,而且与样品的密度或孔隙度有关.     

激光辐照对MgB_2超导体电子结构的影响

研究了光辐照对MgB2超导体电子结构的影响。对MgB2超导体进行不同时间的激光照射后,利用正电子湮没技术对光照样品进行了研究,根据正电子寿命谱参数的变化情况,对光辐照后MgB2超导体样品中缺陷及电子结构的变化进行了讨论。实验表明:MgB2超导体中的平均电子浓度对激光的照射十分敏感,并随光照时间的增加而增加。     

化合物半导体材料的正电子寿命计算

在局域密度理论（LDA）和广义梯度理论（GGA）的基础上计算了ZnO,GaN,GaAs,SiC和InP五种化合物半导体材料中的正电子湮没信息,包括化合物半导体材料中的自由态正电子的湮没寿命;还有不同类型空位（单空位,双空位）附近俘获的束缚态正电子密度分布和湮没率分布,以及束缚态正电子的湮没寿命. 关键词： 半导体 正电子寿命     

La2-xBaxCuO4超导体的PAT研究

本文报道了在室温下对La2-xBaxCuO4超导系列样品的正电子寿命谱（PAT）测量，讨论了正电子湮没机制与超导转变温度的关联，并从电子密度转移方面解释了La2-xBaxCuO4超导样品在1／8处的转变温度受抑制的原因．     

Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：     

Doppler程序对金红石型TiO2的空位缺陷研究

金红石型TiO2是一种非常好的稀磁半导体材料,其自身的本征缺陷与室温铁磁性起源密切相关。本文利用Doppler程序在广义梯度理论（GGA）的基础上,计算了正电子在金红石型TiO2块材中不同缺陷处的湮没寿命。主要包括自由态正电子的湮没寿命,单空位和双空位处束缚态正电子的湮没寿命。并从理论上给出了含有空位缺陷时金红石型TiO2的符合多普勒展宽能谱。     

用正电子湮没技术研究形变铁的恢复

本文介绍应用正电子湮没寿命和多普勒加宽方法研究60％形变铁等时退火各阶段中缺陷的恢复行为,实验结果说明正电子湮没平均寿命τ及多普勒加宽的S参数都是退火温度的函数,完整晶体中湮没寿命τ_f=111±lPS,缺陷中湮没寿命τ_d=162±lps,本文还根据捕获模型计算了正电子的捕获速率及τ₁,结果证明与理论预期的完全一致,经计算得出捕获正电于的相对缺陷浓度的范围为10^-7-10^-4数量级。 关键词：     

LaBaCuO系超导体1/8反常现象及正电子寿命谱的研究

我们在室温下对La2-xBaxCuO4(x=0.05,0.1,0.125,0.15,0.2)超导系列样品的正电子寿命谱(PAT)进行了测量,分析了正电子湮没机制与超导转变温度之间的关联,并从电子密度转移方面对La2-xBaxCuO4超导样品的转变温度被抑制的原因进行了解释.     

HgCdTe中特征正电子湮没寿命值的确定

提出了一种新的在热平衡状态下实时测量碲镉汞(MCT)中特征正电子湮没寿命的方法,用该方法测得HgCdTe中基体正电子湮没寿命τ_b为277±1Ps,汞空位缺陷捕获态寿命τ_d为306±2ps,并与用其它方法测量的结果进行了比较与讨论.     

热处理温度对MgB2超导材料的超导电性及微观结构的影响

采用粉末烧结方法在不同温度（650℃，700℃，750℃，800℃，850℃，900℃）制备了MgB2超导块材．采用X射线衍射（XRD）和扫描电镜（SEM）研究了热处理温度对MgB2超导材料的成相及微结构的影响．采用磁化法测定不同温度制备MgB2超导材料的超导电性．结果显示热处理温度对MgB2超导材料的晶粒尺寸、形状和超导电性有明显影响，700℃制备的MgB2超导体具有最高的临界电流密度和最好的磁通钉扎特性，细小的晶粒尺寸是样品磁通钉扎特性改善的原因．     

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.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.