Temperature dependent exchange bias effect in polycrystalline BiFeO3/FM (FM = NiFe, Co) bilayers

Extensive studies on the temperature (T) dependent exchange bias effect were carried out in polycrystalline BiFeO₃(BFO)/NiFe and BFO/Co bilayers. In contrast to single-crystalline BFO/ferromagnet (FM) bilayers, sharp increase of the exchange bias field (H _E ) below 50 K were clearly observed in both of these two bilayers. However, when T is higher than 50 K, H _E increases with T and decreases further when T is larger than 230 K (for BFO/NiFe) or 200 K (for BFO/Co), which is similar to those reported in single-crystalline BFO/FM bilayers. After the exploration of magnetic field cooling, the temperature dependent exchange bias can be explained considering two contributions from both the interfacial spin-glass-like frustrated spins and the polycrystalline grains in the BFO layer. Moreover, obvious exchange bias training effect can be observed at both 5 K and room temperature and the corresponding results can be well fitted based on a recently proposed theoretical model taking into account the energy dissipation of the AFM layer.     

梯度计算的集合变分方案及其在大气Ekman层湍流系数反演中的应用

大气Ekman层湍流系数的准确计算对数值天气预报和污染物扩散计算有着重要的意义. 将集合计算和变分法结合起来, 提出了目标泛函梯度计算的集合变分方案, 并根据正演模式的线性化情况提出了两种计算流程. 利用这种集合变分梯度算法及两种流程对Ekman层湍流系数进行了反演试验, 结果表明这种算法实施简单、方便, 根据观测资料能够比较准确地反演湍流系数值. 关键词： 梯度计算 集合变分 湍流系数 反演     

光纤放大器受激布里渊散射效应抑制

提出了一种多波长窄线宽光纤放大器,其种子光由多个单频激光耦合而成,所有单频激光波长几乎相等（波长间隔小于1 nm）,频率间隔大于两倍布里渊频移。建立了此类多波长窄线宽光纤放大器的完整理论模型,分析了放大器中受激布里渊散射（SBS）与种子数目的关系。搭建了双波长和三波长窄线宽光纤放大器,进行SBS阈值输出功率测定实验。实验结果与理论模拟结果基本一致,验证了理论模型的合理性;双波长和三波长放大可以有效抑制SBS效应,大幅提高放大器输出功率。     

MgB2超导圆线基底交流损耗的计算分析

二硼化镁(MgB2)超导体在非铜氧化物陶瓷超导体中具有最高临界转变温度,给超导应用带来了新的契机.铁与镁不反应,且价格低廉,在需要热处理的超导成材工艺中,常常采用铁作MgB2的基底;然而,这种高磁导率的材料有可能增大基底的损耗.本文采用有限元分析软件ANSYS计算了工频下不同材料基底超导圆线传输交流的基底损耗,分析了基底损耗与电流大小、基底厚度之间的关系,对双层基底的损耗也进行了研究.本文的结果对于适当选取截面尺寸、基底材料和工作电流以期减小交流损耗,具有一定参考意义.     

光子晶体中四能级系统的量子相干效应

研究了处于光子带隙材料中的四能级原子系统的电磁感应透明、自发辐射和光子开关效应，分析了其稳态与瞬态特性, 发现特殊的模密度能够导致反常的吸收、色散、自发辐射及瞬态无反转增益, 并可以通过外加调制场进行控制.详细讨论了特殊频率处模密度的变化对透明窗口和光子开关效应的影响. 关键词： 光子带隙材料 电磁感应透明 模密度 光子开关     

Structural and optical properties of ZnO film by plasma-assisted MOCVD

High quality ZnO film was deposited by plasma-assisted metal-organic chemical vapor deposition (MOCVD). We observed a dominant peak at 34.6° due to (0 0 2) ZnO, which indicated that the growth of ZnO film was strongly C-oriented. The full-width at half-maximum (FWHM) of the -rocking curve was 0.56° indicating relatively small mosaicity. Photoluminescence (PL) measurement was performed at both room temperature and low temperature. Ultraviolet (UV) emission at 3.30 eV was found with high intensity at room temperature while the deep level transition was weakly observed at 2.513 eV. The ratio of the intensity of UV emission to that of deep level emission was as high as 193, which implied high quality of ZnO film. From PL spectrum at 10 K, we observed A-exciton emission at 3.377 eV and D°X bound exciton transition at 3.370 eV. The donor–acceptor transition and LO phonon replicas were observed at 3.333 and 3.241 eV respectively. Raman scattering was performed in back scattering at room temperature. The E₂, A_1(LO) and A_1(TO) mode was seen at 437.6, 575.8 and 380 cm^–1 respectively. In comparison with Raman spectrum of ZnO powder, we found that ZnO film was nearly free of strain, which indicated high crystal quality.     

面心立方晶体外延膜沉积生长中失配位错的结构与形成过程

用分子动力学方法对5%负失配条件下面心立方晶体铝薄膜的原子沉积外延生长进行了三维模拟.铝原子间的相互作用采用嵌入原子法(EAM)多体势计算.模拟结果再现了失配位错的形成现象.分析表明，失配位错在形成之初即呈现为Shockley扩展位错，即由两个伯格斯矢量为〈211〉/6的部分位错和其间的堆垛层错组成，两个部分位错的间距、即层错宽度为1.8 nm,与理论计算结果一致；外延晶体薄膜沉积生长中，位错对会发生滑移，但其间距保持稳定.进一步观察发现，该扩展位错产生于一种类似于“局部熔融-重结晶”的表层局部无序紊乱- 关键词： 失配位错 外延生长 薄膜 分子动力学 铝     

Millimeter wave beams diffracted from reflecting gratings

According to the angular-spectrum method, a radiation beam of an antenna horn in the Fresnal region is decomposed into the plane waves with the fast Fourier transform algorithm and the waves diffracted from a reflection grating are superposed as a diffracted beam. Compared with the reflected beam from the same size mirror, the radiation half width of the diffracted beam from a grating is narrower and its lateral shape is shift. These performances have been experimentally verified at Ka-band by: (1). The relative diffraction efficiencies in the first order with two triangular gratings which is put along propagating direction of a beam produced by a conical lens-horn. (2). H-plane lateral width with power—3dB lapsed of the focus beam from a grating and a mirror in a grating spectrometer for millimeter waves (25–100 GHz).     

THE APPLICATION OF HIGH RESOLUTION SOLID STATE NMR IN ADSORPTION SYSTEMS

High resolution solid state NMR techniques(such as MAS,CP/MAS andCRAMPS)were employed to study the nature of organic molecules adsorbed onporous solids. A magic angle spinning system was achieved for sealed samples with a spinning speed from 2KHz to 4.2KHz.Using this technique,high resolution ~1HMAS spectra of organic molecules and H_2S adsorbed on charcoal were obtained.EXperimental results suggest that for high coverage of adsorbed organic molecules,the spectral lines were resolved very well.But for low coverage,the spectrallines could not be separated completely.As the organic molecules condensed in     

自由电子激光光导的数值模拟与分析

从一维自由电子激光理论出发,唯象地考虑衍射效应,计算和分析了光导效应的特点和规律;针对曙光一号装置的自由电子激光参数,利用二维理论对光场剖面、填充因子、光场半径等进行了数值模拟;根据计算结果分析和讨论了影响光导的因素和实验条件。