铁磁/有机半导体/铁磁系统的电流自旋极化性质研究

根据有机半导体中的电流自旋极化注入和输运实验现象，理论上研究了铁磁/有机半导体/铁磁系统的电流自旋极化性质.考虑到有机半导体的具体特性，从自旋扩散理论和欧姆定律出发，得到了系统的电流自旋极化率.假设自旋极化子和不带自旋的双极化子为有机半导体中的载流子.通过计算发现，极化子为实现有机半导体中电流极化注入和输运的有效自旋载流子，即使它只占总载流子很少一部分.还进一步研究了自旋相关界面电阻和电导率匹配以及有机半导体长度等因素对系统电流自旋极化的影响. 关键词： 自旋电子学 自旋注入 有机半导体 极化子     

局域交换场和电场调控的锗烯纳米带自旋过滤效应

基于非平衡格林函数方法,理论研究了Z轴方向局域交换场和电场对锗烯纳米带中电子自旋极化输运性质的影响.结果表明对锗烯的边缘区域施加强度大于其2倍有效自旋轨道耦合强度的交换场,可使自旋向上和向下电子的能带在不同的能量区间产生带隙,从而实现对不同自旋取向电子的100%过滤.提出了一种利用电场辅助降低自旋过滤效应所需阈值交换场强度的方法.研究表明,同时对中心器件区域施加局域交换场和电场,可以在小于有效自旋轨道耦合的弱交换场强度下,在较大的能量窗口区域过滤自旋向上或向下的电子.增大局域交换场强度,自旋过滤效应所对应的能量区间显著增大.     

门电压控制的硅烯量子线中电子输运性质

硅烯是由单层硅原子形成的二维蜂窝状晶格结构,具有石墨烯类似的电学性质,由于硅烯中存在比较强的自旋轨道耦合而备受关注.本文利用非平衡格林函数方法研究了门电压控制的硅烯量子线中电子输运性质和能带结构.研究发现,只有在较强的门电压下,而且硅烯量子线具有较好的锯齿形或扶手椅形边界而不存在额外硅原子时,硅烯量子线中才存在无能隙的自旋极化边缘态.另外,计算结果表明这种门电压控制的硅烯量子线中边缘态在每个能谷处自旋是极化的.这些计算结果将为实验上利用电场制作硅烯纳米结构提供理论支持.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

电子的谷自由度

电子在晶格周期性势场影响下的运动遵循布洛赫定理. 布洛赫电子除了具有电荷和自旋两个内禀自由度外, 还有其他内禀自由度. 能带色散曲线上的某些极值点作为谷自由度, 具有独特的电子结构和运动规律. 本文从布洛赫电子的谷自由度出发, 简单介绍传统半导体的谷电子性质研究现状, 并重点介绍新型二维材料体系, 如石墨烯、硅烯、硫族化合物等材料中谷相关的物理特性. 有效利用谷自由度的新奇输运特性, 将其作为信息的载体可以制作出新颖的纳米光电子器件, 并有望造就下一代纳电子器件的新领域, 即谷电子学(valleytronics).     

极化子比率变化对自旋注入有机半导体性质的影响

从自旋扩散方程和欧姆定律出发研究了铁磁层到有机半导体的自旋注入,得到了系统的电流自旋极化率。有机半导体中的载流子为自旋极化子和不带自旋的双极化子,极化子比率在有机半导体内随输运距离变化。通过计算发现匹配的铁磁和有机半导体电导率有利于自旋注入;通过调节界面电阻自旋相关性,电流自旋极化率可获得很大程度提高;极化子比率衰减速率对有机半导体电流自旋极化率具有非常重要的影响。     

扰动对有机磁体器件自旋极化输运特性的影响

基于紧束缚模型和格林函数方法,研究了有机磁体晶格扰动和侧基自旋取向扰动对金属/有机磁体/金属三明治结构有机自旋器件自旋极化输运特性的影响.计算结果表明:晶格扰动的存在降低了器件的起始偏压,减小了导通电流,并使得电流-电压曲线的量子台阶效应不再显著,扰动不太强时电流仍呈现较高的自旋极化率;而侧基自旋取向扰动减小了体系的自旋劈裂,增加了器件的起始偏压,低偏压下随着扰动的增强器件电流及其自旋极化率明显降低.进一步模拟了温度对器件自旋极化输运的影响. 关键词： 有机自旋电子学 有机磁体 自旋极化输运 自旋过滤     

逾渗驱动的高自旋极化氧化物材料磁电阻增强效应——网络效应与调控

锰氧化物具有内禀的多尺度非均匀性,这与同时活跃的多个自由度--自旋、电荷、晶格和轨道--非线性耦合,以及多种相互作用的共存密切相关.更重要的是, 这种极为特殊的物理现象可能是庞磁电阻效应的微观起源--铁磁金属相在磁场作用下的逾渗而驱动的磁电阻效应.另一方面,在某组分导电逾渗阈值附近的磁电阻显著增强效应,是高自旋极化氧化物颗粒体系所具有的普遍现象之一.因此,针对各种高自旋极化氧化物的非均匀和颗粒复合体系,逾渗驱动磁电阻增强效应的研究具有重要的学术意义和应用价值,其中输运网络理论为重要的理论研究.在充分认识电磁输运微观机制的基础之上,通过调控输运网络的结构,探讨逾渗驱动磁电阻增强的必要条件,可以找出实现可控性高且幅值较大的磁电阻的新途径、新方法.本文主要基于电阻网络模型,综述高自旋极化氧化物材料中多相共存体系的磁输运性质研究的主要背景和发展现状,充分结合相关的实验结果,介绍逾渗驱动磁电阻效应增强的物理机制,以及各类电输运网络的构建,并展望未来的发展.     

纳米管中的自旋极化

结构缺陷、掺杂等可以导致纳米管的自旋极化，而自旋极化的纳米管可以利用载流子的自旋状态作为信息载体，实现一维的自旋电子传输。具有不同自旋极化结构的纳米管，根据其电子结构、自旋极化性质和输运性质的不同，可以用于实现不同用途的自旋电子器件。很多有关自旋极化的纳米管的理论和实验工作已经展开，然而其中仍有很多物理问题有待深入研究。     

逾渗驱动的高自旋极化氧化物材料磁电阻增强效应——网络效应与调控

锰氧化物具有内禀的多尺度非均匀性,这与同时活跃的多个自由度——自旋、电荷、晶格和轨道——非线性耦合,以及多种相互作用的共存密切相关。更重要的是,这种极为特殊的物理现象可能是庞磁电阻效应的微观起源——铁磁金属相在磁场作用下的逾渗而驱动的磁电阻效应。另一方面,在某组分导电逾渗阈值附近的磁电阻显著增强效应,是高自旋极化氧化物颗粒体系所具有的普遍现象之一。因此,针对各种高自旋极化氧化物的非均匀和颗粒复合体系,逾渗驱动磁电阻增强效应的研究具有重要的学术意义和应用价值,其中输运网络理论为重要的理论研究。在充分认识电磁输运微观机制的基础之上,通过调控输运网络的结构,探讨逾渗驱动磁电阻增强的必要条件,可以找出实现可控性高且幅值较大的磁电阻的新途径、新方法。本文主要基于电阻网络模型,综述高自旋极化氧化物材料中多相共存体系的磁输运性质研究的主要背景和发展现状,充分结合相关的实验结果,介绍逾渗驱动磁电阻效应增强的物理机制,以及各类电输运网络的构建,并展望未来的发展。     

Current-induced pseudospin polarization in silicene

The pseudospin polarization induced by an external electric field in silicene in the presence of weakly spinindependent impurities is considered theoretically in the linear response regime based on Green’s function method. We study the effects of the interplay between the sublattice potential and the intrinsic spin orbit coupling on the pseudospin polarization. We show that the pseudospin polarization perpendicular to the electric field is independent of the impurity parameter, while the pseudospin polarization in the direction of the electric field is sensitive to the impurity parameter. The dependences of the pseudospin polarizations on the chemical potential are studied.     

Valley-dependent electron transport in ferromagnetic/normal/ferromagnetic silicene junction

The electron transport through ferromagnetic/normal/ferromagnetic silicene junction with an induced energy gap is investigated in this work. The energy gap can be tuned by applying electric field or exchange fields due to the buckled structure of silicene. We analyze the local electric field, exchange field, length of normal region-dependence transmission probabilities of four groups and valley conductance. These transmission probabilities and valley conductance can be turned on or off by adjusting the local electric field and exchange field. In particular, a fully valley polarized conductance with 80% transmission is found in this junction, which can be caused by the interplay of valley-dependent massive Dirac electron, the exchange potential and the on-site potential difference of sublattices. Our findings will benefit applications in silicene-based high performance nano-electronics.     

Electrical control of valley and spin polarized current and tunneling magnetoresistance in a silicene-based magnetic tunnel junction

We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation.     

Modulation of electronic properties with external fields in silicene-based nanostructures

This work reviews our recent works about the density functional theory(DFT) calculational aspects of electronic properties in silicene-based nanostructures with the modulation of external fields, such as electric field, strain, etc. For the two-dimensional(2D) silicene-based nonostructures, the magnetic moment of Fe-doped silicene shows a sharp jump at a threshold electric field, which indicates a good switching effect, implying potential applications as a magnetoelectric(ME) diode. With the electric field, the good controllability and sharp switching of the magnetism may offer a potential applications in the ME devices. For the one-dimensional(1D) nanostructures, the silicene nanoribbons with sawtooth edges(SSi NRs) are more stable than the zigzag silicene nanoribbons(ZSiNRs) and show spin-semiconducting features. Under external electric field or uniaxial compressive strain, the gapless spin-semiconductors are gained, which is significant in designing qubits for quantum computing in spintronics. The superlattice structures of silicene-based armchair nanoribbons(ASiSLs) is another example for 1D silicene nanostructures. The band structures of ASi SLs can be modulated by the size and strain of the superlattices. With the stain increased, the related energy gaps of ASi SLs will change, which are significantly different with that of the constituent nanoribbons. The results suggest potential applications in designing quantum wells.     

Controllable spin transport in dual-gated silicene

Based on the dual-gated silicene, we have evaluated theoretically the spin-dependent transport in lateral resonant tunneling structure. By aligning the completely valley-polarized beam with spin-resolved well state in concerned structure, large spin polarization can be expected owing to spin-dependent resonant tunneling mechanism. Under the gate electric field modulation, the forming quantum well state can be externally manipulated, triggering further the emergence of externally-controllable spin polarization. Importantly, integrating the considered structure with a proper valley-filter, which might be constructed from valley-contrasting physics as that in graphene valleytronics, completely-polarized spin beam can also be attained without the assistance of ferromagnetic component, providing thus some profitable strategies to develop nonmagnetic spintronic devices residing on silicene.     

Dc conductance of ordered and disordered silicene superlattices

This paper studies the conductance of charge carriers through silicene-based superlattices consisting of monolayer silicene by means of transfer matrix method. At first, we consider the ordered superlattices and drive analytically the transmission probability of Dirac fermions. We show that the number of resonance picks increases with increasing the number of superlattice barriers. In order to the best understand of the appearance of the picks, we exactly studied transmission properties of the silicene superlattice. Also, the effect of disorder on the probability of transmission through the system of various sizes is studied. The short-range correlated disorder is applied on the thickness of electron doped silicene strips as quantum barriers which fluctuates around their mean values. We show that the oscillating conductance as a function of barriers hight suppresses with imposing the disorder in the silicene superlattice. Also, the effect of structural parameters on the conductance of the system is studied.     

Mode selective control of flow turbulence

We study spin transport in normal/ferromagnetic/normal/ferromagnetic.../normal graphene superlattices, which can be realized by putting a series of magnetic insulator bars on top of a graphene sheet. Owing to magnetic proximity effect, local exchange splittings will be induced in the graphene sheet, effectively forming a magnetic graphene superlattice. The spin polarization of tunneling conductance and the magneto resistance (MR) exhibit oscillatory behavior with the gate voltage. The superlattice structure leads to an enhanced spin polarization and MR ratio, making the magnetic graphene superlattice become very promising in spintronics applications.     

Transport and Conductance in Fibonacci Graphene Superlattices with Electric and Magnetic Potentials

We investigate the electron transport and conductance properties in Fibonacci quasi-periodic graphene superlattices with electrostatic barriers and magnetic vector potentials.It is found that a new Dirac point appears in the band structure of graphene superlattice and the position of the Dirac point is exactly located at the energy corresponding to the zero-averaged wave number.The magnetic and electric potentials modify the energy band structure and transmission spectrum in entirely diverse ways.In addition,the angular-dependent transmission is blocked by the potential barriers at certain incident angles due to the appearance of the evanescent states.The effects of lattice constants and different potentials on angular-averaged conductance are also discussed.     

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF₂/ZnF₂)/FeF₂.     

Goos–H?nchen-like shift related to spin and valley polarization in ferromagnetic silicene

We study the Goos–H?nchen-like shift of single silicene barrier under the external perpendicular electric field, offresonant circularly polarized light and the exchange field modulation using the stationary-phase method. The results show that the Goos–H?nchen-like shift of silicene resulting from the external perpendicular electric field does not have the characteristics of spin or valley polarization, while that from off-resonant circularly polarized light or the exchange field is spin-polarized. More importantly, the combined effect of the external perpendicular electric field and the exchange field or off-resonant circularly polarized light can cause the Goos–H?nchen-like shift of the system to be spin and valley polarized.It is particularly worth noting that when the three modulations are considered at the same time, as the exchange field changes, the system will have a positive or negative Goos–H?nchen-like shift.