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.     

La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/Ag_x中的室温磁电阻增强

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2(x=0.00,0.04,0.08,0.25,0.30)样品,通过M～T曲线,ρ～T曲线,ρ～T拟合曲线,研究样品的磁性质、输运行为、输运机制及磁电阻效应.结果表明:少量掺杂时Ag可能参与反应.掺杂量较多时,Ag主要以金属态分离到母体颗粒的界面处,使体系形成两相复合体.少量的Ag掺杂可以明显提高自旋相关散射产生的晶界磁电阻.掺Ag为30%摩尔比时,样品的电阻率较低掺杂样品的电阻率降低一个数量级,在300K、0.5T磁场下,磁电阻明显增强,达到9.4%,这与颗粒母体界面结构的改善有关,也与材料电阻率的降低有关.     

共沉淀法制备纳米多晶La2／3Sr1／3MnO3及其低场磁电阻性质

利用共沉淀法制备了纳米多晶La2/3Sr1/3MnO3粉体,并研究了相应烧结陶瓷的相结构、显微形貌、电性质以及磁电阻效应.样品中存在高密的晶界,对电子的疏运过程产生明显的散射作用,从而使得样品具有较大的电阻值,并且只有当烧结温度大于800℃时才出现金属绝缘体温度相变.样品的磁电阻是低场磁电阻效应,源于晶界处电子的自旋极化隧穿.烧结温度越低的样品,其磁电阻越大.     

纳米晶类钙钛矿型锰氧化物导电性的高压研究

研究了纳米晶类钙钛矿型锰氧化物La0.7Sr0.3Mn0.95Fe0.05O3和La0.7Ca0.3MnO3在不同压力、高温处理后的导电性质.结果表明,在室温高压成型样品具有很高的电阻率,居里温度TC附近磁电阻效应很小 随处理压力的增加,电阻率先是降低,至11GPa时达最低点,之后升高.高温处理过的样品经一定压力处理后,其电阻随温度变化曲线的金属绝缘体转变峰向低温跃变,并伴随电阻率的大幅增加. 关键词：     

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.     

SrTiO3掺杂La-K-Mn-O系统的电磁性质

用溶胶-凝胶制备了La0.833K0.167MnO3-SrTiO3(LKMO/STO)系列样品,并研究了它们的结构、磁性和输运特性.X射线衍射实验表明,1200℃烧结的LKMO/STO(STLK12)是一个均匀的固溶相.其电阻率表现为绝缘体的行为,而纯La0.833K0.167MnO3(LKMO)样品随温度的升高则有金属-绝缘体转变.在低场下(μ0H=0.02 T),对STLK12样品,当温度从220 K降低到4 K时,磁电阻从0.2%升高到11%.在高场下(μ0H=5.5 T),随着温度降低,磁电阻几乎是线性增大.在4.2 K时,达到65%.比纯LKMO样品40%的磁电阻高出了25%.我们用晶界处的自旋极化隧穿效应定性地解释了这种增强的磁电阻效应.     

添加Ag的Nd0.7Sr0.3MnO3薄膜研究

本文研究了Ag的添加对超大磁阻材料Nd0.7Sr0.3MnO3薄膜磁电特性的影响。通过对相同条件下制备的Nd0.7Sr0.3MnO3薄膜和添加Ag的Nd0.7Sr0.3MnO3薄膜的测量，着重比较了电阻温度系数（TCR）和磁电阻（MR）的变化。所制备的添加Ag的Nd0.7Sr0.3MnO3薄膜经过高温退火处理后，在1Tesla下得到较高的磁电阻MR值，其电阻温度系数TCR也大为增加。     

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.     

Ag和K掺杂对La4/5Sr1/5MnO3电输运性质及磁电阻的影响

用固相反应法制备La4/5Sr1/5MnO3及在其A位分别掺K、Ag系列样品,通过X射线衍射(XRD)谱,电阻率-温度(ρ～T)曲线,磁电阻-温度(MR～T)曲线,研究了在A位同时掺入一价、二价元素而保持Mn3+/Mn4+比值(摩尔比n(A)/n(B))不变的钙钛矿锰氧化物体系A位离子半径及A位离子的无序度σ2对电输运性质及磁电阻的影响.结果表明:A位离子的无序度σ2对电输运性质的影响比A位平均离子半径对电输运性质的影响大;电阻率曲线出现双峰是由于表面相电阻率与体相电阻率竞争的结果;MR的温度稳定性是本征磁电阻与隧穿磁电阻竞争的结果;掺K样品在253～175K温区MR从8.1%缓慢上升到9.5%,掺Ag样品在260K以下温区MR都在7.4%以上,纯的La4/5Sr1/5MnO3样品在318～259K温区MR都在7.0%以上,在如此宽温区MR几乎不变有利于MR的实际应用.     

(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构制备及表征

采用射频磁控溅射的方法在SrTiO3(001,基片上制备了(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构.X射线衍射分析证明(La0.7Sr0.3MnO3)m(BiFeO3)n具有明显的超晶格结构.电流垂直于薄膜表面测得的电阻-温度关系表明.La0.7Sr0.3MnO3)m(BiFeO3)n超晶格薄膜在290 K有金属-绝缘体转变,略低于单层La0.7Sr0.3MnO3薄膜的转变温度.电流在0.01-10 mA范围内,观察到薄膜的峰值电阻随电流增大而减小,峰值变化率远大于单层La0.7Sr0.3MnO3薄膜,且随着超晶格周期厚度的增加而增大.低温下,电流-电压曲线表明其导电机制应主要为空间载子限制,且显示较大的电压偏置,表现出肖特基结的特性.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

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.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

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.     

Research on beamforming method for dual-line array with space mismatch between the two line arrays

The performance of port/starboard discrimination of dual-line array will be seriously degraded when the space between the two line arrays of the dual-line array is mismatched. The directivity function of the delay-sum and delay-subtract beamforming is deduced and the port/starboard discrimination of the delay-sum and delay-subtract beamforming is also studied both under the ideal status and the status under which the space between the two line arrays of the dual-line array is mismatched. Analysis results show the delay-subtract beamforming has better performance of port/starboard discrimination. At last, the sea trial results demonstrate the theoretical results.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Retrieval of ultraviolet skylight radiances and O3 slant column densities from balloon-borne limb spectrometer

With a novel light-weight and absolutely calibrated ultraviolet （UV） spectrometer, UV skylight radiances and O3 slant column densities are measured by balloon-borne limb measurements in Xinjiang area, China. UV skylight radiances measured at the height of 31 km are compared with the results from Modtran in the wavelength range from 290 to 420 nm. O3 slant column densities are derived from radiance spectra in the Huggins bands （320 - 335 nm） using differential optical absorption spectroscopy method. And the parameter exhibits a good correlation with the same value simulated by radiative transfer model （Tracy）. The O3 profile simultaneously measured by an O3 sonde is used as input in Tracy calculations. The O3 sonde is launched on the same balloon.