有机自旋阀的磁电阻性质研究

考虑到有机半导体中极化子和双极化子特殊的电荷-自旋关系,从自旋扩散方程和欧姆定律出发,理论研究了"铁磁/有机半导体/铁磁"有机自旋阀结构中的磁电阻性质.计算发现,磁电阻在数值上随有机半导体层中极化子比率的增加而增大,随有机半导体层厚度的增加而迅速减小.同时发现自旋相关界面电阻能在很大程度上提高系统的磁电阻.讨论了铁磁层和有机半导体电导率比率、铁磁层极化率等对系统磁电阻性质的影响. 关键词： 磁电阻 有机自旋电子学 极化子     

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     

自旋注入有机物的扩散理论

根据自旋注入半导体的相关理论， 考虑到有机体内可能同时含有带自旋的单极化子和不带自旋的双极化子两种载流子，从扩散 理论和欧姆定律出发，建立了自旋注入有机体的唯象模型.通过计算发现，适当选择铁磁层极化率或两层的电导率可以使得有机层内电流具有高的自旋极化.进一步研究了单极化子浓度等因素对注入电流极化的影响. 关键词： 自旋电子学 自旋注入 有机聚合物 极化子     

极化子和双极化子在齐聚物中的稳定性

导电聚合物中的元激发稳定性问题对于认识和理解有机材料中的自旋极化输运现象是极其重要的.针对目前存在的极化子与双极化子之间的争论,本文从半经验的Austin Model 1(AM1)方法出发,对齐分子噻吩的掺杂态进行了研究.通过对极化子和双极化子体系能量的比较,发现双极化子可以转换为极化子,从而揭示出在有机半导体材料中可以实现自旋极化输运.     

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。     

掺杂锰氧化物和极化子行为

掺杂锰氧化物属于强关联材料,实验上明确显示出强电-声子耦合对于材料性质(如输运性质和铁磁性质)的影响.极化子是对氧化物材料中载流子行为的近似描述.顺磁状态的掺杂锰氧化物样品电阻率随温度变化关系可以用极化子理论进行很好的描述,并且数据分析还提供了与材料性质有关的微观参数.利用文章作者提出的跃迁极化子交换相互作用与材料铁磁有序温度TC联系的关系式,对掺杂锰氧化物样品磁场下的测量结果进行了数据拟合.非常好的拟合结果说明,跃迁极化子决定交换耦合及铁磁有序的物理图像和数据拟合中使用的近似是合理的.在数据拟合基础上对掺杂锰氧化物和巨磁电阻(CMR)效应的物理图像进行了讨论.     

钙钛矿锰氧化物中的极化子研究

钙钛矿锰氧化物(以下简称锰氧化物,如La_1-xSr_xMnO₃等,x为掺杂浓度)因其优异的电、磁性质受到人们广泛的关注,但是对于其材料内部载流子性质的认识至今仍没有统一定论.本文基于锰氧化物内Mn—O链的特点,建立一维紧束缚模型,对锰氧化物载流子的性质展开研究.发现在掺杂浓度x=0.5时,系统处于铁磁态,自旋能级完全劈裂,价带和导带之间存在带隙,所有电子态呈现扩展行为.进一步掺杂,将出现局域电子态,同时伴随着晶格的局域畸变,形成所谓的极化子.伴随着极化子的出现,带隙中出现极化子深能级.极化子携带的电荷量越多,形成的晶格缺陷越深,局域能级也越深.当极化子的电荷量继续增加时,极化子解离,载流子倾向于形成能量更低的正反"孤子"对.     

共轭聚合物中链内无序效应对极化子输运的影响

基于扩展的Su-Schrieffer-Heeger紧束缚模型, 利用非绝热动力学方法研究了链内无序效应对共轭聚合物中极化子输运机制的影响. 研究发现, 极化子的输运由外加电场和链内无序效应共同作用的结果所决定. 在一般情况下, 链内无序效应不利于极化子的输运, 但随着电场强度的增大, 无序对极化子输运的影响减小. 关键词： 共轭聚合物 极化子输运 链内无序     

二维链间扩展的极化子动力学研究

基于扩展的SSH模型,研究了有序耦合聚合物链系统中的极化子动力学,包括极化子的形成过程及其在外场下的输运.发现,当聚合物链间的耦合较强时,注入到系统中的电子会诱发二维链间扩展的极化子态,分布在多条聚合物链上.另外,动力学模拟表明,与一维链内定域极化子相比,在相同的电场强度下二维极化子具有更大的运动速度,这与实验结果一致. 关键词： 链间扩展极化子 链间耦合     

链间耦合对极化子非弹性散射性质的影响

基于一维紧束缚的Su-Schreiffer-Heeger模型, 采用非绝热动力学方法, 研究了链间耦合对聚合物中极化子对非弹性散射性质的影响: 激子的产生依赖于链间耦合, 随着耦合强度的增加, 正负极化子对的电子波函数交叠增强, 利于提高激子的产率; 当耦合区域是极化子的宽度时, 正负极化子对波函数的耦合最充分、耦合最强, 电荷跃迁更容易, 激子产率最大. 关键词： 聚合物 极化子 激子     

Theoretical study on spin-polarized injection of electrical currents into polymer semiconductors

Different from electrons and holes in traditional inorganic semiconductors, the charge carriers in polymer semiconductors are spin polarons and spinless bipolarons. In this paper, a theoretical model is presented to describe the spin-polarized injection of electrical currents from a ferromagnetic contact into a nonmagnetic polymer semiconductor. In this model, a new relation of conductivity to concentration polarization for polymer semiconductors is introduced based on a three-channel model to describe the spin-polarized injection of electrical currents under large electrical current densities. The calculated results of the model reveal the effects of the polaron ratio, the carrier concentration polarization, the interfacial conductance, the bulk conductivity of materials, and the electrical current density, etc. on the spin polarization of electrical currents. As conclusions, the large and matched bulk conductivity of materials, the small spin-dependent interfacial conductance, the thin polymer thickness and the large enough electrical current are critical factors for upgrading the spin polarization of electrical currents in polymer semiconductors. Particularly, when the polaron ratio in polymer semiconductors approaches the concentration polarization of the ferromagnetic contact, a modest concentration polarization is sufficient for achieving a nearly complete spin-polarized injection of electrical currents.     

Magnetization reversal and spin accumulation in nonmagnetic metals by spin injection from lateral Co/Cu,Co/Al heterojunctions

We have fabricated ferromagnetic/nonmagnetic (FM/NM) metal heterojunctions for the detection of the spin accumulation effect in different nonmagnetic metals. To understand the effect of spin accumulation in more detail, the switching behavior of the ferromagnetic wires was studied by means of magnetoresistance (MR) measurements and Monte Carlo simulations (MC). The polycrystalline heterojunctions were prepared by high-resolution electron beam lithography (HR-EBL) and a special oblique evaporation technique. The ferromagnetic (FM) and the nonmagnetic (NM) metal were evaporated on top of each other in a single-evaporation step to achieve an interface between the two metals of high quality. To verify the quality of the interface, we measured the spin accumulation effect in nonmagnetic copper (Cu) and aluminum (Al) and determined the spin polarization of the current at the interface between the ferromagnetic and nonmagnetic metals.     

A two-leg spin ladder model for fumarate-bridged chain-like polymer of Cu(II)

In this paper, the transfer matrix renormalization group method is used to study the thermodynamic and the magnetic properties of a novel one-dimensional fumarate-bridged Cu(II) chain by a two kinds of models. One simplified model is that of the dominant ferromagnetic and antiferromagnetic alternating chain, which is only in rough agreement with the experimental results. Further research shows a possible mechanism of alternating strong ferromagnetic and weak antiferromagnetic interaction among one chain. Considering the weak antiferromagnetic interaction existing in the actual materials, we propose a two-leg spin ladder model with intrachain ferromagnetic and weak interchain antiferromagnetic interaction. The obtained theoretical results are in quite good agreement with experimental curves, which indicates the two-leg spin ladder model is appropriate to describe the fumarate-bridged chain-like polymer of Cu(II).     

Universal behaviors of magnon-mediated spin transport in disordered nonmagnetic metal-ferromagnetic insulator heterostructures

We numerically investigate magnon-mediated spin transport through nonmagnetic metal/ferromagnetic insulator (NM/FI) heterostructures in the presence of Anderson disorder, and discover universal behaviors of the spin conductance in both one-dimensional (1D) and 2D systems. In the localized regime, the variance of logarithmic spin conductance σ²(lnG_T) shows a universal linear scaling with its average ⟨lnG_T⟩, independent of Fermi energy, temperature, and system size in both 1D and 2D cases. In 2D, the competition between disorder-enhanced density of states at the NM/FI interface and disorder-suppressed spin transport leads to a non-monotonic dependence of average spin conductance on the disorder strength. As a result, in the metallic regime, average spin conductance is enhanced by disorder, and a new linear scaling between spin conductance fluctuation rms(G_T) and average spin conductance G_T is revealed which is universal at large system width. These universal scaling behaviors suggest that spin transport mediated by magnon in disordered 2D NM/FI systems belongs to a new universality class, different from that of charge conductance in 2D normal metal systems.     

Brillouin-zone mapping of the spin waves in a ferromagnetic bilayer system

The eigenproblems of spin waves in a heterogeneous ferromagnetic bilayer system with periodic boundary conditions are solved using the interface-rescaling approach. Brillouin zone mapping and the eigenmodes of the system are investigated. We find three types of spin waves may exist in the system: the bulk mode, the interface mode, and the perfect confined mode. The fine structure of the energy band in the heterogeneous bilayer system is first given for the whole two-dimensional Brillouin zone. Conditions for the existence of the interface mode are discussed. Finally, we analyze the resonant-confined spin waves in bulk modes and their oscillating behavior.     

Theory of spin polarization in mesoscopic spin–orbit coupling systems

We establish a general formalism of the bulk spin polarization (BSP) and the current-based spin polarization (CSP) for mesoscopic ferromagnetic and spin–orbit interaction (SOI) semiconducting systems. Based on this formalism, we reveal the basic properties of BSP and CSP and their relationships. The BSP describes the intrinsic spin polarized properties of devices. The CSP depends on both intrinsic parameters of device and the incident current. For the non-spin-polarized incident current with the in-phase spin-phase coherence, CSP equals to BSP. We give analytically the BSP and CSP of several typical nanodevice models, ferromagnetic nanowire, Rashba nanowire and rings. These results provide basic physical behaviors of BSP and CSP and their relationships.     

The competition between CDW and SDW in quasi-one-dimensional organic magnetic polymer with interchain coupling interaction

Based on a theoretical model proposed for interchain-coupled quasi-one-dimensional organic magnetic polymer, the effects of the interchain couplings and electron–electron interactions on the charge density wave (CDW) and spin density wave (SDW) that exist in the system are studied. It is found that the amplitude of the SDW along the main chain will decrease with increasing of the oscillatory term of the interchain couplings in the system, which is unfavorable to the ferromagnetic ground state of the system. Moreover, with different interchain couplings, there will all exist a critical value of the inter-site electron–electron Coulomb repulsion, and at this value, the system will experience a transformation from strong SDW state to strong CDW one, which will weaken the mediating function of the antiferromagnetic SDW along the main chain. As a result, the ferromagnetic correlation intensity between the spins of the side radicals will be affected and consequently the stability of the ferromagnetic state in the system will be weakened.     

Quantum spin and charge pumping through double quantum dots with ferromagnetic leads

The pumping of electrons through double quantum dots (DQDs) attached to ferromagnetic leads have been theoretically investigated by using the nonequilibrium Green?s function method. It is found that an oscillating electric field applied to the quantum dot may give rise to the pumped charge and spin currents. In the case that both leads are ferromagnet, a pure spin current can be generated in the antiparallel magnetization configuration, where no net charge current exists. The possibility of manipulating the pumped spin current is explored by tuning the dot level and the ac field. By making use of various tunings, the magnitude and direction of the pumped spin current can be well controlled. For the case that only one lead is ferromagnetic, both of the charge and spin currents can be pumped and flow in opposite directions on the average. The control of the magnitude and direction of the pumped charge and spin currents is also discussed by means of the magnetic flux threading through the DQD ring.