Transition from tunneling leakage current to molecular tunneling in single-molecule junctions

数十年来，半导体工业一直遵循基于“摩尔定律”所设定的发展蓝图，逐步提升集成电路芯片上晶体管的集成度和运行速度，减小器件尺寸。为探索这一尺寸极限，课题组基于机械可控裂结技术自主开发了具有飞安级电学测量和亚纳米级位移控制灵敏度的科学仪器，在国际上首次获取了一系列具有不同重复单元的寡聚苯乙炔类分子电导随电极间距的演变关系，并发现随着电极间距的缩小，器件电输运由通过分子器件电流占主导逐步转变到由隧穿漏电流占主导。对于本研究中具有最小尺寸的寡聚苯乙炔分子器件，其由于隧穿漏电流所制约的尺寸极限可小至0.66 nm，预示了有机分子器件在未来电子器件小型化方面具有重要的应用潜力。 这一研究工作是在化学化工学院洪文晶教授、萨本栋微纳研究院杨扬助理教授以及英国Durham University的MartinR. Bryce教授共同指导下完成的。能源材料化学协同创新中心iChEM Fellow刘俊扬博士为论文第一作者，博士研究生郑珏婷、李瑞豪和硕士研究生黄晓艳、唐永翔、皮九婵、本科生王飞等参与了研究工作。田中群教授、毛秉伟教授和师佳副教授为论文工作提供了重要指导。【Abstract】The tunneling leakage current will be a major quantum obstacle during miniaturization in the semiconductor industry down to the scale of several nanometers. At this scale, to promote charge transport and overcome the tunneling leakage current between the source and drain terminals, molecular electronic junctions offer opportunities by inserting molecules between these two electrodes. Employing a series of oligo(aryleneethynylene) (OAE) molecules, here we investigate the transition from tunneling leakage current to molecular tunneling in the single-molecule devices using mechanically controllable break junction (MCBJ) technique, and the transition distances of the OAE molecular junctions were determined and even down to 0.66 nm for OAE2 molecular junction, which demonstrates that the intrinsic charge transport properties of a single-molecule device can be outstripped from the tunneling leakage current. Consequently, molecular electronic devices show the potential to push the ultimate limit of miniaturization to the scale of several angstroms.This work was supported by the National Key R&D Program of China (2017YFA0204902). This work was also generously supported by the Young Thousand Talent Project of China, the EC FP7 ITN “MOLESCO” project number 606728, the National Natural Science Foundation of China (nos. 21703188, 21673195, 21503179), and the China Postdoctoral Science Foundation (2017M622060). 该工作获得科技部国家重点研发计划课题（2017YFA0204902），国家自然科学基金委（21673195、21703188、21503179）以及中国博士后科学基金（2017M622060）等项目的资助，也得到了固体表面物理化学国家重点实验室、能源材料化学协同创新中心的支持

First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring

In a companion paper, we present the first spatially resolved polarized image of Sagittarius A* on event horizon scales, captured using the Event Horizon Telescope, a global very long baseline interferometric array operating at a wavelength of 1.3 mm. Here we interpret this image using both simple analytic models and numerical general relativistic magnetohydrodynamic (GRMHD) simulations. The large spatially resolved linear polarization fraction (24%–28%, peaking at ∼40%) is the most stringent constraint on parameter space, disfavoring models that are too Faraday depolarized. Similar to our studies of M87*, polarimetric constraints reinforce a preference for GRMHD models with dynamically important magnetic fields. Although the spiral morphology of the polarization pattern is known to constrain the spin and inclination angle, the time-variable rotation measure (RM) of Sgr A* (equivalent to ≈46° ± 12° rotation at 228 GHz) limits its present utility as a constraint. If we attribute the RM to internal Faraday rotation, then the motion of accreting material is inferred to be counterclockwise, contrary to inferences based on historical polarized flares, and no model satisfies all polarimetric and total intensity constraints. On the other hand, if we attribute the mean RM to an external Faraday screen, then the motion of accreting material is inferred to be clockwise, and one model passes all applied total intensity and polarimetric constraints: a model with strong magnetic fields, a spin parameter of 0.94, and an inclination of 150°. We discuss how future 345 GHz and dynamical imaging will mitigate our present uncertainties and provide additional constraints on the black hole and its accretion flow

基于MIMO电力线信道的定时同步算法

针对多输入多输出(MIMO)电力线通信(PLC)系统中的脉冲噪声造成定时同步不准确的问题，文章提出了一种基于MIMO PLC系统的正交频分复用(OFDM)符号定时同步算法，利用差分相关和最大比合并(MRC)提高同步性能。在2×2 MIMO PLC信道模型下，两个发送端口发送自相关性和互相关性良好的前导序列，在接收端通过设置阈值过滤电力线中的脉冲噪声，采用改进的延时自相关算法实现定时粗同步。利用窗口求和方法减小精同步的估计范围后，通过差分相关和MRC联合算法完成定时精同步。相比互相关算法，所提算法有2 dB的提升。仿真结果表明，该方法同步性能明显优于传统互相关算法

十字轴式万向联轴器传动性能分析

以大型养路机械的十字轴式万向联轴器为研究对象，应用运动学理论分析了当十字轴式万向联轴器的输入轴做匀速转动时，其输出轴做圆周性不均匀转动的性能。分别从角速比、不均匀系数和角加速度等方面对十字轴式万向联轴器定量表达了其不均匀性。结果表明，随着十字轴式万向联轴器输入、输出轴夹角、转速的增大，其输出轴最大角加速度增大，使其最大惯性力矩增大。十字轴式万向联轴器的输入、输出轴夹角是影响其不均匀性的关键参数，当十字轴式万向联轴器输入、输出轴夹角增大时，其输出轴转角波动性增大。减小输入、输出轴夹角是提高十字轴式万向联轴器传动效率的唯一途径

气候变化和人类活动在塔里木河流域水文要素中的反映

塔里木河流域较明显的增湿出现在近20 a,流域平原地区近10 a略有暖干化的迹象;20世纪90年代是流域山区近40 a来最暖湿的阶段,在天山南麓中西部山区和帕米尔高原一带90年代增湿幅度大,西昆仑山北坡一带近20 a降水变化很小。塔里木河流域4条源流出山口多年平均径流量为227.0×108m3,从年代际尺度看,50-80年代基本接近多年平均值,而90年代受山区增暖变湿影响,4条源流径流量达241.9×108m3,增幅6.6%。由于源流区人类活动的影响和粗放型农业,近50 a来补给塔里木河干流的源流条数和水量不断减少,加之塔里木河干流上中游区间耗水量大,导致下游水量减少,生态急剧恶化