第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB₂的电子能带结构、声子谱及电声子耦合常数,并讨论MgB₂的超导电性.通过比较MgB₂薄膜双轴拉伸前后超导电性的变化可以看出，随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E_2g声子频率显著下降，使得电声子耦合强度λ和声子对数平均频率ω_ln增强,提高了MgB₂薄 关键词： 超导电性 能带结构 声子频率 电声子耦合     

DFT Study on Structural Distortion and Vibronic Coupling of Vanadyl Porphyrin Anion and Cation

The geometries of one-electron reduced/oxidized species （[TOP]-/[VOP] ＋） of vanadyl por- phyrin （VOP） have been calculated with PBE1PBE method. The results show that for both [VOP]- and [VOP]＋ the ground states are triplet, in which one of the two unpaired electron occupies the dxy orbital of the V atom while the other occupies the n-orbital of porphyrin ring. Thus both [VOP]- and [VOP]＋ can be considered as n-radicals. The ground state of neutral VOP molecule is doublet with the unpaired electron occupying dxy orbital of V atom. In contract to the C4v symmetry of neutral VOP molecule, [VOP]- anion has a ＂rectangular＂ distorted C2v structure due to Jahn-Teller effect. The linear vibronic coupling constants for the Jahn-Teller active modes of [TOP]- were evaluated and the node patterns of frontier KS orbitals are used to explain the reason why the distortion occurs along specific modes. The ground state [VOP]＋ has a porphyrin ring with pronounced bond length alternation due to pseudo-Jahn-Teller effect, causing its symmetry declined from C4v to Ca. The bond length alternation is well explained with the node patterns of re-constructed frontier KS orbitals.     

超声技术的基石——超声换能器的原理及设计

超声换能器是在超声频率范围内将交变的电信号转换成声信号或者将外界声场中的声信号转换为电信号的能量转换器件,它是超声技术中的关键器件,其性能好坏直接关系到超声应用技术的效果和使用范围.由于超声技术的应用范围很广,且超声新技术层出不穷,因而与此对应的超声换能器的种类也很多.文章对不同应用背景下多种类型超声换能器的原理及设计进行了阐述,分析了不同类型超声换能器的性能参数及设计要求,简要总结了超声换能器的性能参数测试方法,并对超声换能器的发展趋势进行了一定的分析.     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

Electric-Current-Induced Heat Generation in Kondo Regime

Using the nonequilibrium Green＇s function technique, we investigate the current induced heat generation in Kondo regime. The Kondo effect influences the heat generation significantly. In the curve of heat generation versus the bias, a negative differential of the heat generation is exhibited. The symmetry of the heat generation is destroyed by the strong electron-electron interaction and the electron-phonon interaction.     

AlB2型WB2超导电性的第一性原理研究北大核心CSCD

采用基于密度泛函微扰理论的第一性原理方法研究了WB_2型WB_2和AlB_2型WB_2的带心声子性质,计算了AlB_2型WB_2的电声耦合作用和超导转变温度.结果表明,AlB_2型WB_2的电声耦合常数为0.87.电声耦合作用主要来源于B原子振动声子关联模式的贡献.估算得到AlB_2型WB_2的超导温度为16~22.2K(取μ~*=0.15~0.1时).研究结果表明AlB_2型WB_2是一类较强的电声耦合超导体.     

β型烧绿石结构氧化物超导体AOs2O6(A=K,Rb,Cs)电子能带结构的第一性原理计算

用全势线性缀加平面波方法计算β型烧绿石结构氧化物超导体AOs2O6(A=K,Rb,Cs)的电子能带结构及态密度.计算发现电子自旋轨道耦合和在位库仑势U的作用增大了费米面处态密度值.通过计算还得到这三种化合物电子关联常数λc分别为1.55,1.12和0.73.由实验测量与能带计算得到的电子比热容系数的比值得到电子质量提高参数.通过分析这三种化合物电子质量提高参数,推算出它们的电声子耦合常数λep分别为1.56,0.78和1.08.由此提出KOs2O6为强电子关联和强电声子耦合系统,而RbOs2O6和CsOs2O6的电子关联性与电声子耦合为中等.     

Phonon States and Dispersive Spectra of Polar Optical Phonons in Quasi-One-Dimensional Nanowires of Wurtzite ZnO and Zinc-Blend MgO Semiconductors

Within the framework of the macroscopic dielectric continuum model and Loudon's uniaxial crystal model, the phonon modes of a wurtzite/zinc-blende one-dimensional (1D) cylindrical nanowire (NW) are derived and studied. The analytical phonon states of phonon modes are given. It is found that there exist two types of polar phonon modes, i.e. interface optical (IO) phonon modes and the quasi-confined (QC) phonon modes existing in 1D wurtzite/zinc-blende NWs. Via the standard procedure of field quantization, the Fröhlich electron-phonon interaction Hamiltonians are obtained. Numerical calculations of dispersive behavior of these phonon modes on a wurtzite/zinc-blende ZnO/MgO NW are performed. The frequency ranges of the IO and QC phonon modes of the ZnO/MgO NWs are analyzed and discussed. It is found that the IO modes only exist in one frequency range, while QC modes may appear in three frequency ranges. The dispersive properties of the IO and QC modes on the free wave-number k_z and the azimuthal quantum number m arediscussed. The analytical Hamiltonians of electron-phonon interaction obtained here are quite useful for further investigating phonon influence on optoelectronics properties of wurtzite/zinc-blende 1D NW structures.     

铅基弛豫型铁电单晶研究进展及其应用

铅基弛豫型铁电单晶由于其具有潜在的巨大商业和军用价值日益受到了人们的重视.本文主要对新型压电单晶的研究进展进行了较为全面的概述.并对其在电声换能技术中的应用前景进行了分析.同时指出了目前研究中存在的主要问题并提出展望.     

高效室温行波热声制冷机的研究(第二部分:电声压缩机驱动的行波热声制冷机的实验研究)

热声制冷机是利用热声效应将声能转化为热能的新型制冷装置 ,以其环保性及高可靠性等优点吸引着起来越多的科研者。该文研究对象为电声压缩机驱动的室温行波热声制冷机。首先介绍了制冷机的工作原理、设计及加工 ;实验系统建立后 ,改装曲柄 -连杆活塞压缩机 ,使之提供变频的波动压力作为该制冷机的驱动源。初步实验在充气压力 2 .0 MPa、氦气为工质的工况下 ,获得 2 6 4 K的制冷温度