Burst模式在多波束声呐高帧率测深方法中的应用

在常规多波束测深声呐系统中,需要等待远距离目标信号到达接收基阵后才能再次发射探测信号,这将导致测深帧率下降。针对该问题,本文提出了一种基于Burst模式的多波束声呐高帧率测深方法。由于各角度的海底回波持续时间远小于远距离目标信号到达接收基阵的所需时间,根据这一特性我们采用Burst模式,通过等间隔地发射多帧信号并接收,同时保证发射的间隔大于最大波束持续时间,实现多帧信号在时间角度域中的分离,提高测深系统帧率。由于相邻帧的隧道效应会与部分波束发生重叠,针对该问题采用改进的线性约束最小方差算法,实现了低副瓣宽零陷的波束形成,从而削弱隧道效应的影响。最后通过仿真数据验证了本文所提方法的有效性,表明该方法在特定条件下能够将测深帧率提高3.3倍。     

典型类比分析法的原理与实践

本文以我国隧道工程应用新奥法的典型工程经验和原位测试资料为基础,综合应用系统科学、工程地质力学和岩石力学方法,阐述在隧道工程锚喷支护设计与施工中,普及应用围岩稳定分析预测技术的原理和方法—典型类比分析法的提出、原理框架与工程验证。     

Large positive and negative magnetoresistance in armchair phosphorene nanoribbons

We investigate the tunnel magnetoresistance (TMR) in an armchair phosphorene nanoribbon modulated by two ferromagnetic stripes. It is shown that large TMR can be achieved by applying a perpendicularly electric field to the phosphorene plane. We find that the TMR can be adjusted by an external gate voltage, and the TMR oscillates periodically from positive to negative by a slight change of the gate voltage. This characteristic can be observed in a wide region of exchange splitting values. Our findings open the way to design phosphorene-based spintronics nanodevices, and it may contribute to the future low power spintronic applications.     

各向异性光子晶体的全反射隧穿偏振特性

为了研究一维各向异性光子晶体的全反射隧穿偏振特性,利用传输矩阵法计算了TE波和TM波以大于全反射角入射一维各向异性光子晶体时的透射率,在透射波中发现了TE波和TM波的一、二级全反射隧穿带.得出了与一维各向同性光子晶体的全反射隧穿效应的不同点,即TE波和TM波的一、二级全反射隧穿带的频率中心和频率宽度存在着较大的差异,利用这个特性可以将两种偏振光进行频率分离,为设计优质偏振滤波器提供理论依据.     

深埋偏压小净距隧道施工力学特征数值模拟研究

以瑞赣高速公路峡山隧道区间为工程背景, 针对小净距双洞并行隧道的复杂 受力状态, 采用有限元法对深埋条件下的小净距段偏压隧道采用不同施工方案进行数值模拟. 对双侧壁导坑法、CD法和上下台阶法等不同的施工方法对围岩应力、围岩变形、锚喷支护体 系结构内力、拱顶位移及围岩稳定性进行对比. 分析不同方案下开挖前、后隧道围岩与支护 结构的位移、应力变化规律. 在此基础上对不同方案施工过程对隧道中间夹岩的力学行为进 行分析, 提出了隧道中间夹岩薄弱面的概念, 更加明确了该工程地质条件下隧道夹岩的薄弱 控制面. 并以此为依据指导工程施工, 在实际施工中取得了较好的效果.     

在不同形状的防沙墙下沙纹形状及分布分析

论述了在塔里木盆地皮山-沙雅连线西北的塔克拉玛干沙漠西北边缘地带的流动沙漠区域进行的现场风洞试验.在试验中采用了四种形状的防沙墙,分别为直线型、半圆型、三角型、M字型.观测和测试了用不同形状的防沙墙时,沙波形成和性质的改变,同时进行了对比分析,为今后的计算机数值模拟提供了试验依据.     

Theoretical estimates of optimal parameters for quantum cascade lasers

The quantum cascade (QC) laser is a new light source which is based on one type of carrier (electrons) making transitions between energy levels created by quantum confinement. In this paper, focusing on the working conditions which a QC laser should satisfy, we have discussed the subband lifespans in QC laser active regions. The results show that the population inversion condition can be achieved by resonant tunneling associated with an optical phonon, and this population inversion can be facilitated by the short escaping time of electrons from one active region to the neighboring active region. Our calculations also show that the lifespans of levels 3 and 2 are dominated by the phonon scattering time, and the escaping time from one active region to the next active region is determined by the thickness of exit barrier and the proper design of the miniband between the active regions.     

Spin valve effect and negative tunnel magnetoresistance in a quantum dot transistor

We study the spin polarized transport through a quantum dot transistor. It is shown that the interplay of large Coulomb interaction and optically induced spin accumulation gives rise to the spin valve effect over a range of bias. We also find negative tunnel magnetoresistance for system with ferromagnetic electrodes.