La0.5Sm0.2Sr0.3MnO3/x（Sb2O3）体系电输运性质及MR温度稳定性

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/x(Sb2O3)(x=0.00,0.02,0.04,0.05,0.075,0.10,0.15)系列样品,通过X射线衍射(XRD)谱、电阻率-温度(ρ～T)曲线、ρ～T拟合曲线、磁电阻-温度(MR～T)曲线,研究了该体系的电输运性质及MR的温度稳定性.所有样品的电输运性质都表现出绝缘体-金属相变,相变温度很高(312K)且基本保持不变,随Sb2O3复合量增大,电阻率迅速增大,类金属导电可以用ρ=ρ0+AT2公式拟合,表明导电机制是电子-电子相互作用,x=0.075的样品,在200～320K温区磁电阻基本保持不变,MR的温度稳定性是晶界引起的隧穿磁电阻与钙钛矿颗粒体相本征磁电阻竞争的结果.     

用脉冲激光方法在Si(100)上沉积的Cox-C1-x颗粒膜及其磁电阻效应

利用脉冲激光沉积方法在Si(100)上制备了Cox-C1-x颗粒膜,并研究了其正磁电阻效应.实验结果表明,样品在室温下的正磁电阻效应要远远高于低温下的正磁电阻效应;Co0.02-C0.98样品具有最大的室温正磁电阻效应,在外加磁场B=1T时,其磁电阻率MR=22%;随着Co含量的增加,Cox-C1-x颗粒膜的正磁电阻效应呈减小趋势.样品的MR-B的曲线与传统的多层膜及颗粒膜结构有很大的不同,这一现象表明在实验样品中可能存在着一种新的输运机制.     

(La_(1-x)Ce_x)_(2/3)Ca_(1/3)MnO_3体系的结构和输运性质研究

对Ce掺杂锰氧化物 (La1 -xCex) 2 3Ca1 3MnO3(x =0— 1.0 )多晶样品的结构和输运性质系统研究的结果 .实验表明 ,在低掺杂浓度下 ,样品呈现完整的正交钙钛矿结构 ;随掺杂浓度的增加 ,有少量CeO2 杂相出现 ,同时伴随有Ce3 和Ce4 离子两种价态的涨落和Mn2 Mn3 Mn4 混合价态的共存 ,Ce掺杂导致的体系无序度增加 ,使得绝缘体 -金属 (I M)和顺磁 -铁磁 (PM FM)转变温度向低温方向移动 .有趣的是 ,Ce掺杂样品的电阻率测量I M转变峰值温度TIM 较PM FM转变温度Tc 为高 ,其差值ΔT(=TIM -Tc)随Ce掺杂含量的增加而增加 ,最大差值ΔT达 5 0K .各样品的磁电阻则随温度的降低而增加 ,在Tc 附近MR达到最大值 ,且随Ce掺杂含量增加Tc 附近的MR最大值迅速增加 ,达到 10 4 %以上 ,表现出很强的庞磁电阻效应 .在x≥ 0 .7时 ,磁电阻效应则表现出反常减小 .整体上而言 ,各样品的磁特性与输运行为间有较强的关联 ,电输运特性可用双交换作用模型进行很好的解释 ,在高温区满足热激活模型 .最后 ,在Ce掺杂对Tc 和MR的影响机理方面进行了初步讨论 .     

In替代(La2/3Ca1/3)(Mn(3-2x)/3In2x/3)O3体系巨磁电阻效应的研究

采用固态反应法制备了In替代的(La_2/3Ca_1/3)(Mn_(3-2x)/3In_2x/3)O₃(x=0.00，0.10，0.15)体系.通过测量其零场和1.6T磁场下样品的电阻-温度关系以及一定温度下磁电阻率与磁场的关系.发现随In³⁺替代量的增加其磁电阻峰和电阻峰均向低温方向移动，同时巨磁电阻效应减弱，磁电阻峰也展宽.这是由于In³⁺替代量的变化，引起 关键词：     

Ag复合对La2/3(Ca0.45Sr0.55)1/3MnO3/xAg体系电输运和室温磁电阻效应影响

文中采用甘氨酸-硝酸盐法分别制备了La2/3(Ca0.45Sr0.55)1/3MnO3/xAg纳米复合材料(x=0;0.1;0.2;0.3).通过X射线衍射、扫描电子显微镜和磁电阻效应测试,对合成产物的结构及性能进行表征.结果表明,随Ag复合量增加,样品均为正交钙钛矿结构,低场室温磁电阻效应增强,电阻率减小.     

(La1-xYx)2/3Ca1/3MnO3(x=0.195)系统中的磁电阻行为

本文实验上探讨了 (La1-xYx)2/3Ca1/3MnO3(x=0.195)窄带系统不同磁场下的磁电阻行为.实验表明,最大磁电阻效应随磁场增加而实质性提高,例如,7T磁场下最大磁电阻效应高达1.8×105%.同时,我们在远远低于半导体-金属转变温度下也观察到明显的磁电阻效应.文中就磁电阻的起因进行了讨论,特别是基于最近提出的自旋-极化子模型,对实验数据进行了定量的比较性分析,结果表明,该模型虽能预言实验的主要特征,但理论预言随磁场增加而明显偏离实验.     

Ag掺杂La-K-Mn-O非均匀多晶体系的电磁性质

用溶胶-凝胶方法制备了1/mAg₂O-La_0.833K_0.167MnO₃ (LKMO/Ag)系列样品，其中1/m代表Ag₂O和La_0.833K_0.167MnO₃(LKMO)的摩尔比，m=32，16，8，4和2. 研究了此系列样品的结构、磁性和输运特性. X射线衍射实验表明，LKMO/Ag是一个非均匀的系统，样品由磁性的钙钛矿相LKMO和金属Ag相组成. 由于Ag相的加入，在室温条件下，磁电阻效应明显增强. 在300 K, 0.5 T磁场下，m=4样品的磁电阻可以达到32%；5.5 T磁场下，其磁电阻可达64%. 而单纯的LKMO样品在相同条件下的磁电阻分别为10%和35%. 在低温下，加Ag样品的磁电阻效应反而减小，样品含Ag越多，磁电阻效应越小. 用非本征磁电阻(包括自旋极化隧穿和自旋相关散射)和本征磁电阻在不同温区对总磁电阻的相对贡献对此系列样品的磁电阻现象作了定性的解释. 关键词： 自旋极化隧穿 自旋相关散射 低场磁电阻 高场磁电阻     

La0.5Ca0.5-xBaxMnO3体系中的异常磁电阻行为

实验研究了La0.5Ca0.5MnO3系统Ca位上Ba掺杂对其磁电阻效应的影响.结果表明,掺杂量低于0.2的样品,磁电阻效应随温度降低而增大,在转变温度附近达到最大值,然后随温度降低而变小,随着温度进一步降低磁电阻效应再次增大.而对于掺杂量高于0.2的样品,磁电阻效应在整个温区随温度降低而单调增加.文中通过考虑铁磁性和反铁磁性两种效应的竞争而对实验观察给出解释.     

液氦温区La2/3Ca1/3Mn1-xCuxO3(x=0.15)体系的磁电阻弛豫效应

实验上研究了液氦温区La2/3Ca1/3Mn1-xCuxO3(x=0.15)单相多晶样品给定磁场下的电阻随时间的变化,发现磁电阻随时间变化并非遵从对数规律而是呈指数型变化行为.对这一指数型磁电阻弛豫行为的可能起因进行了讨论,指出这是与Mn位上Cu掺杂将额外磁性杂质引入到样品中有关.     

用脉冲激光方法在Si(100)上沉积的Cox-C1-x颗粒膜及其磁电阻效 应

利用脉冲激光沉积方法在Si(100)上制备了Co_x-C_1-x颗粒膜，并研 究了其正磁电阻 效应.实验结果表明，样品在室温下的正磁电阻效应要远远高于低温下的正磁电阻效应；Co _0.02-C_0.98样品具有最大的室温正磁电阻效应，在外加磁场B=1T时 ，其磁电阻 率MR=22%；随着Co含量的增加，Co_x-C_1-x颗粒膜的正磁电阻效应呈 减小趋势.样品 的MR-B的曲线与传统的多 关键词： 正磁电阻效应 x-C_1-x颗粒膜')" href="#">Co_x-C_1-x颗粒膜 脉冲激光沉积     

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.