NMR evidence of charge fluctuations in multiferroic CuBr_2

We report combined magnetic susceptibility, dielectric constant, nuclear quadruple resonance(NQR), and zero-field nuclear magnetic resonance(NMR) measurements on single crystals of multiferroics CuBr_2. High quality of the sample is demonstrated by the sharp magnetic and magnetic-driven ferroelectric transition at T_N= T_C≈ 74 K. The zero-field ~(79)Br and ~(81)Br NMR are resolved below T_N. The spin-lattice relaxation rates reveal charge fluctuations when cooled below 60 K.Evidences of an increase of NMR linewidth, a reduction of dielectric constant, and an increase of magnetic susceptibility are also seen at low temperatures. These data suggest an emergent instability which competes with the spiral magnetic ordering and the ferroelectricity. Candidate mechanisms are discussed based on the quasi-one-dimensional nature of the magnetic system.     

Bi-2223/Ag高温超导带临界电流各向异性的实验测量及分析

本文介绍了Bi-2223/Ag高温超导带临界电流各向异性的实验测量方法及原理,指出了在不同磁场方向(与超导带面夹角)下,Bi-2223/Ag带临界电流随直流背景磁场的变化规律以及实验应注意的问题.Bi-2223/Ag带材的各向异性决定了其临界电流对垂直于带材表面的磁场非常敏感,这是该带材在高温超导磁体等强电应用方面的不利因素.     

光子晶体微纳传感技术的理论与实验研究进展

光子晶体(PC)微纳传感器具有体积小、易集成、响应时间快、控光能力强、可实现无标签检测以及设计灵活等优点,在工业生产、海洋探测、生物医疗检测及环境监测等领域展现出巨大的应用前景。针对基于一维纳米束PC与二维平板PC的微纳光子传感技术,从传感器结构设计和传感性能提升方面,分析了当前国内外PC微纳传感的关键理论和实验技术进展,并通过对比分析,探讨了PC微纳传感技术在未来应用中所面临的挑战及未来发展的趋势。     

YBCO堆叠高温超导磁体的磁悬浮振动传递特性实验研究

在YBCO堆叠磁体与永磁轨道构成的高温超导磁悬浮系统中,永磁轨道的振动通过电磁作用传递给YBCO堆叠磁体.本文通过实验研究了振动在YBCO堆叠磁体磁悬浮系统中的传递效率与基振加速度幅值和基振频率的关系.研究表明:在YBCO堆叠磁体磁悬浮系统中,YBCO堆叠磁体自由振动频率在9 Hz左右;一定低频条件下,振动的传递效率与基振加速度幅值呈正相关趋势,与基振频率呈负相关趋势.     

Femtosecond electron pulse compression by using the time focusing technique in ultrafast electron diffraction

We present a new model of an electron gun for generating subrelativistic femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with a time focusing region, the time focusing electrode generates an electron energy chirp for bunching at the target. Without considering the space charge effects, simulations of the electron gun were carried out under the conditions of different dc voltages and various slopes of the voltage added on the time focusing electrode. Tracing and simulating large numbers of photoelectrons through Monte-Carlo and finite difference methods, the electron pulses with 1 ps can be compressed to 55 fs, which will allow significant advances in the field of ultrafast diagnosis.     

Dopant diffusion and surface morphology of vanadium implanted 4H-silicon carbide

The diffusion behaviours of vanadium implanted p- and n-type 4H-SiC are investigated by using the secondary ion mass spectrometry (SIMS). Significant redistribution, especially out-diffusion of vanadium towards the sample surface is not observed after 1650℃ annealing for both p- and n-type samples. Atomic force microscopy (AFM) is applied to the characterization of surface morphology, indicating the formation of continuous long furrows running in one direction across the wafer surface after 1650℃ annealing. The surface roughness results from the evaporation and re-deposition of Si species on the surface during annealing. The chemical compositions of sample surface are investigated using x-ray photoelectron spectroscopy (XPS). The results of C 1s and Si 2p core-level spectra are presented in detail to demonstrate the evaporation of Si from the wafer and the deposition of SiO₂ on the sample surface during annealing.     

基于乙烯或氢气的吸气式旋转爆轰发动机实验

基于氢气的旋转爆轰发动机研究较多,而碳氢燃料与空气混合较为困难,导致基于乙烯的旋转爆轰发动机燃烧技术难度很高.使用宽视野范围的可视化燃烧室观察旋转爆轰波的研究在国内尚未开展.在同一燃烧室内进一步开展了乙烯或氢气的吸气式旋转爆轰实验,来流总温为283~284 K,燃烧室壁面有140°石英玻璃观察窗,便于观察旋转爆轰波运动过程.空筒燃烧室爆轰环腔外径为100 mm,轴向长度为151 mm.燃料通过150个直径0.8 mm圆柱孔进入燃烧室,空气通过喉部1 mm宽的收敛扩张环缝流入环腔.高速摄影和低高频压力传感器均验证了旋转爆轰波的存在和速度值.以氢气为燃料的旋转爆轰波速度最高可达理论值的101%,爆轰波增压效应可达40%左右,乙烯旋转爆轰波速度可达理论值的89%.旋转爆轰波结构容易发生变化,不规则.氢气旋转爆轰的维持对燃烧室的结构要求比碳氢燃料要低,比乙烯旋转爆轰波更加稳定.      

大功率4K脉管制冷机模拟和实验研究

提高液氦温区脉管制冷机的制冷量对于冷却超导磁体和氦液化具有重要意义。本文将已成功应用于单级脉管制冷机和4 K G-M制冷机模拟的回热器模拟软件REGEN用于液氦温区脉管制冷机二级回热器的模拟。计算结果显示,脉管制冷机在4.2 K的制冷量一定程度上随着二级质量流量的增加而增大。在此基础上,我们在实验中采用不同质量流量的压...     

808 nm大功率半导体激光器阵列的优化设计

 采用激射波长为808 nm的GaAs/AlGaAs梯度折射率波导分别限制单量子阱结构外延片,制备了沟道深度不同的半导体激光器阵列,并对载流子分布进行理论分析和模拟。理论和实验结果表明：引入脊形台面和隔离沟道后,激光器阵列的输出功率、电光转换效率、斜率效率和光谱特性均有显著提高。随着沟道的加深,对电流侧向扩散的限制作用增强,从而提高了阵列性能。     

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应用准光学原理设计了HL-2A装置电子回旋共振加热(ECRH)系统新的集束天线,该天线能使4束68GHz/500kW/1s电子回旋波通过椭球镜聚焦和平面镜的反射,从一个直径350mm装置窗口同时注入托卡马克,对等离子体实现加热。根据基模高斯束的传播原理得出,在装置环向横截面中心处单条波束的功率密度为158MW•m-2,功率密度降为中心密度的1/e2的半径为31.7mm,微波束经过镜面聚焦和反射产生的欧姆损失和衍射损失分别为0.27%和0.64%。利用有限元分析软件Ansys对镜面进行热分析得到,在1s脉冲载荷下最大镜面温升仅为0.47℃,镜面可以自然冷却。