二噻吩硼烷异构体分子结构测定的第一性原理研究

本文在第一性原理计算基础上结合非平衡格林函数方法,研究了量子干涉效应对连接镍电极的二噻吩硼烷(dithienoborepin,DTB)分子结自旋输运性质的影响,并通过氨基和硝基钝化实现了对二噻吩硼烷分子异构体(DTB-A和DTB-B)的区分.结果表明,原始的DTB-A和DTB-B分子结在费米能级两侧都有一个自旋向上透射峰和一个自旋向下透射峰,且两个透射峰的能量位置和高度基本相同.因此,原始DTB-A和DTB-B分子结的自旋向上和自旋向下电流曲线基本重合,不能被明显区分.然而,研究发现量子干涉效应能不同程度地增强氨基钝化DTB-A分子结费米能级两侧分子轨道的自旋极化输运能力,并减弱氨基钝化DTB-B分子结费米能级两侧分子轨道的自旋极化输运能力.此外,研究还发现量子干涉效应可以显著提高硝基钝化DTB-B分子结费米能级两侧分子轨道的自旋极化输运能力,同时减弱硝基钝化DTB-A分子结费米能级两侧分子轨道的自旋极化输运能力.由于量子干涉效应对氨基和硝基钝化的DTB异构体分子结自旋输运能力有不同的调制作用,因此可以通过测量氨基和硝基钝化分子结的自旋电流值来区分DTB分子的两种异构体.     

硅团簇自旋电子器件的理论研究

利用过渡金属掺杂的硅基团簇, 构建了一种自旋分子结; 并利用第一性原理方法, 对其电子自旋极化输运性质进行了研究. 计算表明, 通过过渡金属掺杂可以有效地产生自旋极化电流, 磁性金属Fe和非磁性金属Cr和Mn掺杂的体系呈现出较明显的自旋极化透射现象, 但分子结的自旋极化输运能力与团簇孤立状态下的磁矩无一致性.从Sc到Ni的掺杂, 体系的自旋极化透射能力先增大后迅速减小, 在Fe掺杂的Si₁₂团簇中出现最大值. 关键词： 硅团簇 自旋极化输运 密度泛函理论 非平衡格林函数     

金属-氧化物界面在磁隧道结中的作用

磁隧道结通常是指由两层磁性金属和它们所夹的一层氧化物绝缘层 (Ⅰ )所组成的三明治结构 .通过绝缘层势垒的隧穿电子是自旋极化的 ,这种自旋极化能够反映作为电极的铁磁金属费米能级处态密度 (ＤＯＳ)的变化 .起源于自旋极化的隧道磁阻(ＴＭＲ)效应已成为当代磁学的一个研究热点 .尽管目前在ＴＭＲ的研究和应用方面已有所发展 ,但在理解隧穿自旋极化方面还存在一定的分歧 ,争论的焦点是金属 -氧化物界面对自旋极化的影响 .对于Ｃｏ、Ｎｉ等铁磁金属 ,由于多数自旋电子所处的ｄ带位于费米能级之下 ,理论预期其自旋极化应该是负的 .然而Ｆ/Ａ…     

有机铁磁界面自旋极化接触构型效应

基于第一性原理,研究了三种不同的接触构型垂直吸附在镍表面的苯双硫分子的界面自旋极化.结果表明界面自旋极化强烈依赖于接触构型,接触构型的变化可使自旋极化由正值变为负值.通过分析投影态密度,发现界面处镍原子的3d轨道与硫原子的sp³杂化轨道发生了轨道杂化.模拟机械可控断裂结实验中的界面吸附构型,根据计算的界面自旋极化,利用Julliere模型得到磁电阻约为27%,与实验测量结果较为符合.     

影响磁性pn结自旋极化输运特性的因素

利用漂移扩散理论研究了磁性pn结中自旋的输运特性.探讨了外加电压、平衡自旋极化率、外加自旋注入和自旋寿命对磁性pn结电流密度和电阻的影响,讨论了磁性pn结自旋伏特效应与pn结宽度的关系.发现平衡自旋极化率使得不同自旋方向电子具有不同的势垒高度从而能有效调制电流;而外加自旋注入则为磁性pn结提供了非平衡自旋极化电子从而达到对电流的调制作用,同时发现自旋伏特电流随准中性p区宽度减小而增大. 关键词： 磁性pn结 自旋极化率 自旋寿命 自旋伏特效应     

Λ超子碎裂函数的夸克味分离

利用有非极化和极化的A和A超子产生的中微子和反中微子的深度非弹性散射,可以清晰地测量各种味的夸克变为A超子的各种非极化和极化的碎裂函数,也就是得到了碎裂函数的夸克味分离.这种方法不但适用于轻味夸克。反夸克,也适用于奇异夸克s和s.和产生A和A的极化电子的深度非弹性散射结合起来,人们就能够系统地测量或核实各种味和各种自旋的夸克的相关碎裂函数.这样的测量给有关强子的自旋结构和核子海的夸克一反夸克不对称性的各种预言提供判决性的实验.     

一维强相互作用极化费米子的相图

文章首先简要评述了目前强相互作用的极化冷费米原子体系的研究现状.在三维,人们对该体系基态存在着不同认识.为对这个问题有进一步了解,文章探讨了一维强相互作用极化费米气体.在均匀情况下,这是一个可积系统,可以得到该体系的一个严格相图.作者发现了一种非均匀的超流相在相空间占主导地位.在有外加束缚势的实验情况下,通过局域密度泛函近似,作者发现了两种新颖的相分离相.     

基于光偏振旋转效应的碱金属气室原子极化率测量方法及影响因素分析

利用原子自旋效应能够实现超高灵敏度的惯性和磁场测量。一类操控原子自旋处于无自旋交换弛豫态的器件可以进行物理参数测量。碱金属气室为该类器件的敏感表头。碱金属原子密度与原子极化率是碱金属气室的重要参数,对研究原子自旋处于无自旋交换弛豫态有着重要的作用。光的偏振效应在量子计算和原子物理研究中发挥了重要作用。利用光的偏振效应能够实现对碱金属原子密度与原子极化率的检测。提出一种基于光偏振旋转效应的碱金属原子极化率测量方法。首先对碱金属气室加恒定磁场,利用激光作为检测光,根据光偏振旋转原理,检测通过气室的偏振光的法拉第旋转角,得到碱金属气室原子密度。然后将碱金属原子抽运,利用激光作为检测光,检测通过气室的偏振光的偏转角,得到碱金属原子极化率。该方法在测量原子极化率的过程中也测量了碱金属原子密度,实现利用一套系统测量两个重要参数,具有快速测量和高灵敏度等特点,简化了实验设备及过程。对两种偏转角进行仿真分析,得到该方法实验时检测激光波长变化对偏转角的影响,根据仿真图得到检测激光波长的可取范围,验证了该方法的可行性。最后分析激光器波长波动与磁场波动对其测量精度的影响,提出实验对激光器与磁场的要求。     

利用第一性原理研究Ni掺杂ZnO铁磁性起源

采用基于密度泛函理论和局域密度近似的第一性原理分析了Ni掺杂ZnO磁性质.文中计算了8个不同几何结构的铁磁(FM)和反铁磁耦合能量,结果表明FM耦合更稳定.态密度结果显示Ni 3d 与O 2p发生杂化,导致费米能级附近电子态自旋极化.文中也分析了O空位对Ni掺杂ZnO铁磁性质的影响,O空位通过诱导电子调节FM耦合,从而稳定Ni掺杂ZnO铁磁性质,其强度足以引发室温铁磁性.通过Ni 3d能级耦合具体分析了Ni 掺杂ZnO铁磁性起源.另外,也分析了晶格应变对Ni掺杂ZnO FM耦合的影响. 关键词： 第一性原理 半导体 铁磁性 缺陷     

单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

Spin-dependent transparency of ferromagnet/superconductor interfaces

We combine parameter-free calculations of the transmission and reflection matrices for clean and dirty interfaces with a scattering-theory formulation of Andreev reflection (AR) generalized to spin-polarized systems in order to critically evaluate the use of an extended Blonder-Tinkham-Klapwijk (BTK) model to extract values of the spin polarization for ferromagnetic metals from measurements of point-contact AR. Excellent agreement with the experimental conductance data is found for Pb/Cu but it is less good for Pb/Ni and poor for Pb/Co, indicating that the BTK formalism does not describe transport through superconducting/ferromagnetic interfaces correctly.     

Spin–triplet current in half metal/conical helimagnet/superconductor heterojunctions

The BTK theory is extended to investigate spin–triplet current and differential conductance spectrum in the half metal/conical helimagnet (Holmium)/s-wave superconductor heterojunctions. We show that the effective spin–split and spin–flip scatterings of the Holmium layer control the conversion efficiency between the spin–singlet and equal-spin triplet pair correlations, leading to a tunneling current oscillation with the thickness of the Holmium layer. This can provide qualitative explanations on the current oscillation in Ho/Co/Ho-based Josephson junction experiment. The differential conductance spectrum confirms spin–flip Andreev reflection induced long-ranged equal-spin triplet pair correlations.     

Point-contact tunneling spectroscopy between a Nb tip and an ideal topological insulator Sn-doped Bi_(1.1)Sb_(0.9)Te_2S

The lightly Sn-doped Bi_(1.1)Sb_(0.9)Te_2S is a good material to investigate the pure topological surface state because the bulk bands are far away from the Fermi level. By measuring point-contact tunneling spectra on the topological insulator Bi_(1.08)Sn_(0.02)Sb_(0.9)Te_2S samples with a superconducting Nb tip, we observed the suppression of differential conductance near zero bias, instead of the enhancement due to Andreev reflection on the spectra. The fitting to the measured spectrum results in a superconducting gap of more than 4 meV, and this value is much larger than the superconducting gap of the bulk Nb. The gaped feature exists at temperatures even above the critical temperature of bulk Nb, and is visible when the magnetic field is as large as 9 T at 3 K. We argue that such behaviors may be related to the pressure induced superconductivity by the tip in the junction area, or just some novel phenomena arising from the junction between an s-wave superconductor and an ideal topological insulator.     

Properties of pseudospin polarization on a graphene-based spin singlet superconducting junction

We investigate theoretically transport characteristics in a graphene-based pseudospinmagnet/superconductor junction, including the s-wave and the d-wave pairing symmetry potential in the superconducting region. It is found that the pseudospin polarization, in sharp contrast to spin polarization in the graphene-based ferromagnet/superconductor junction, holds no influence on the specular Andreev reflection for a negligible Fermi energy. Furthermore, the Fano factor is cru-ially affected by the zero bias state. Therefore, we suggest here that the findings could shed light on the realization of graphene-based pseudospintronics devices and provide a new way to detect the specular Andreev reflection and the zero bias state in the actual experiments.     

拓扑绝缘基铁磁/铁磁超导隧道结的磁效应和塞曼效应(英文)

运用拓展的BTK理论研究了拓扑绝缘层上铁磁/铁磁超导隧道结的磁效应和塞曼效应,同时考虑了铁磁体和铁磁超导体之间的费米能级错配效应.研究发现:在该系统中塞曼效应和邻近效应可以共存;铁磁体和铁磁超导体之间的费米能级错配效应能够增强系统中发生在eV=Δ处的Andreev谐振散射过程和邻近效应.     

Coherent quantum transport in normal-metal/d-wave superconductor/normal-metal double tunnel junctions

Taking into account the effects of quantum interference and interface scattering, combining the electron current with hole current contribution to tunnel current,we study the coherent quantum transport in normal-metal/d-wave superconductor/normal-metal (NM/d-wave SC/NM) double tunnel junctions by using extended Blonder-Tinkham-Klapwijk (BTK) approach. It is shown that all quasiparticle transport coefficients and conductance spectrum exhibit oscillating behavior with the energy, in which periodic vanishing of Andreev reflection (AR) above superconducting gap is found.In tunnel limit for the interface scattering strength taken very large, there are a series of bound states of quasiparticles formed in SC.     

Andreev reflexion at nanocontacts of superconducting AuIn2 with copper

We investigated superconducting AuIn₂ by point-contact spectroscopy. Andreev reflexion was observed with nearly full excess current at contact sizes down to 0.15 nm. This implies, together with a large superconducting coherence length of 11 μm, almost ideal retroreflectivity. At the onset of lateral confinement of the ballistic electrons the Andreev-derived spectra broaden dramatically due to inelastic scattering at defects or the strain field near the contact. Lateral confinement does not suppress Andreev reflexion.     

Phase-incoherent superconducting pairs in the normal state of Ba(Fe(1-x)Co(x))₂As₂

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe(0.92)Co(0.08))?As? and a silver tip can be seen in the normal state of the film up to temperature T～1.3T(c), where T(c) is the critical temperature of the superconductor. Andreev reflection far above T(c) can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.     

Specular Andreev reflection in graphene

By combining the Dirac equation of relativistic quantum mechanics with the Bogoliubov-de Gennes equation of superconductivity we investigate the electron-hole conversion at a normal-metal-superconductor interface in graphene. We find that the Andreev reflection of Dirac fermions has several unusual features: (1) the electron and hole occupy different valleys of the band structure; (2) at normal incidence the electron-hole conversion happens with unit efficiency in spite of the large mismatch in Fermi wavelengths at the two sides of the interface; and, most fundamentally: (3) away from normal incidence the reflection angle may be the same as the angle of incidence (retroreflection) or it may be inverted (specular reflection). Specular Andreev reflection dominates in weakly doped graphene, when the Fermi wavelength in the normal region is large compared to the superconducting coherence length.     

Andreev reflexion at nanocontacts of superconducting AuIn2 with copper

We investigated superconducting AuIn₂ by point-contact spectroscopy. Andreev reflexion was observed with nearly full excess current at contact sizes down to 0.15 nm. This implies, together with a large superconducting coherence length of 11 μm, almost ideal retroreflectivity. At the onset of lateral confinement of the ballistic electrons the Andreev-derived spectra broaden dramatically due to inelastic scattering at defects or the strain field near the contact. Lateral confinement does not suppress Andreev reflexion.