Controllable persistent spin-polarized charge current in a Rashba ring

We theoretically predict the appearance of a persistent charge current in a Rashba ring with a normal and a ferromagnetic lead under no external bias. This charge current is the result of the breaking of the time inversion symmetry in the original persistent pure spin current induced by the Rashba spin-orbit coupling (RSOC) in the ring due to the existence of the ferromagnetic lead. With the Keldysh Green''s function technique, we find that not only the magnitude and sign but also the spin polarization of the generated charge current is determined by the system parameters such as the magnetization direction of the ferromagnetic lead, the tunneling coefficient, the strength of the RSOC and the exchange energy of the ferromagnetic lead, which are all tunable in experiments, that is, a controllable persistent spin-polarized charge current can be obtained in such a device.     

三终端多臂量子环中的持续电流

本文提出了三终端多臂量子环模型,并采用特殊方法来计算持续电流,研究发现：总磁通为零时,持续电流随半导体环增大做非周期和不等幅的振荡.总磁通不为零时,持续电流随AB磁通增强做周期性等幅振荡,Rashba自旋轨道耦合具有改变两种持续电流位相和位相差的效应. 平均持续电流的大小与各臂的臂长和所处的位置以及磁通分布相关.AB磁通对两种持续电流具有不同影响,两种持续电流是可分离的.     

Persistent spin currents in a triple-terminal quantum ring with three arms

A new model of a triple-terminal quantum ring with three arms is proposed.We develop an equivalent method for reducing the triple-terminal quantum ring to the double-terminal quantum ring and calculate the persistent spin currents in this model.The results indicate that the persistent spin currents show behavior of nonperiodic and unequal amplitude oscillation with increasing semiconductor ring size when the total magnetic flux is zero. However,when the total magnetic flux is non-zero,the persistent spin currents make periodic equal amplitude oscillations with increasing AB magnetic flux intensity.At the same time,the two kinds of spin state persistent spin currents have the same frequency and amplitude but the inverse phase.In addition,the Rashba spin-orbit interaction affects the phase and the phase difference of the persistent spin currents.The average persistent spin currents relate to the arm length and the terminal position as well as the distribution of the magnetic flux in each arm.Furthermore, our results indicate that the AB magnetic flux has different influences on the two kinds of spin state electrons.     

在外磁场作用下Rashba系统中的磁光吸收谱

基于半经典玻尔兹曼方程的方法研究了InGaAs/InAlAs 系统中电子在Rashba自旋轨道耦合相互作用(RSOI)和外磁场作用下二维电子气(2DEG)的磁光吸收谱以及选择定则。RSOI的存在使朗道能级相互混合并移动, 在高迁移率和强磁场条件下, 磁光吸收谱可以观察到来自相邻郎道能级和相同自旋间的两个主吸收峰。随着电子浓度、外磁场以及自旋轨道耦合强度的不同可以相应地调制吸收谱强度和峰位等。另外, 由于朗道能级的混合, 使磁光谱出现自旋反转的跃迁, 但由于不同自旋态的电子波函数的重叠很小, 此跃迁对磁光吸收谱的贡献很小。     

Rashba效应下三角量子阱中弱耦合束缚极化子的性质

利用线性组合算符和幺正变换相结合的方法,推导出三角量子阱中弱耦合束缚极化子的基态能量。并讨论了耦合常数,库仑束缚势和电子面密度对极化子基态能量的影响。通过对GaAs材料的数值计算,最后结果显示：由于电-声子耦合作用和Rashba效应的存在,弱耦合束缚极化子的基态能量由四部分组成,而且弱耦合束缚极化子的基态能量随耦合常数,库仑束缚势和电子面密度都发生了分裂。     

含Rashba自旋轨道耦合二维电子气结构中电子反常位移及自旋分离

研究了含Rashba自旋轨道耦合的磁电调制半导体二维电子气中弹道电子的反常位移 （Goos-H?nchen位移,即GH位移）。计算中发现,通过调节结构的各个参数包括入射角、磁场强度和Rashba自旋轨道耦合系数,可以有效地调控GH位移,并且在一定条件下可以为负。电子的GH位移和自旋极化态有密切关系,这个自旋相关的位移可以用来分离不同自旋极化的电子束。基于这种现象,提出了一种利用GH位移在半导体2DEG中分离不同自旋极化电子的方法。     

台阶量子阱的结构和电场对Rashba自旋分裂的影响

Spin splitting of conduction subbands in Al0.3Ga0.7As/GaAs/AlxGa1-xAs/Al0.3Ga0.7As step quantum wells induced by interface and electric field related Rashba effects is investigated theoretically by the method of finite difference.The dependence of the spin splitting on the electric field and the well structure,which is controlled by the well width and the step width,is investigated in detail.Without an external electric field,the spin splitting is induced by an interface related Rashba term due to the built-in structure inversion asymmetry.Applying the external electric field to the step QW,the Rashba effect can be enhanced or weakened,depending on the well structure as well as the direction and the magnitude of the electric field.The spin splitting is mainly controlled by the interface related Rashba term under a negative and a stronger positive electric field,and the contribution of the electric field related Rashba term dominates in a small range of a weaker positive electric field.A method to determine the interface parameter is proposed.The results show that the step QWs might be used as spin switches.     

Effect of a step quantum well structure and an electric-field on the Rashba spin splitting

Spin splitting of conduction subbands in Al0.3Ga0.7As/GaAs/AlxGa1-xAs/Al0.3Gaa7As step quantum wells induced by interface and electric field related Rashba effects is investigated theoretically by the method of finite difference. The dependence of the spin splitting on the electric field and the well structure, which is controlled by the well width and the step width, is investigated in detail. Without an external electric field, the spin splitting is induced by an in terface related Rashba term due to the built-in structure inversion asymmetry. Applying the external electric field to the step QW, the Rashba effect can be enhanced or weakened, depending on the well structure as well as the direction and the magnitude of the electric field. The spin splitting is mainly controlled by the interface related Rashba term under a negative and a stronger positive electric field, and the contribution of the electric field related Rashba term dominates in a small range of a weaker positive electric field. A method to determine the interface parameter is proposed. The results show that the step QWs might be used as spin switches.     

（英）嵌有两个量子点的三终端量子环中的横向自旋流

从理论上研究了一个上臂与下臂各嵌有一个量子点的三终端三臂量子环中的自旋极化的电子输运性质.考虑了两个量子点中的库仑互作用并假设在某一个环臂中存在Rashba自旋轨道耦合作用.利用非平衡态格林函数方法,发现不需要借助任何磁场与磁性物质Rashba自旋轨道耦合作用导致的量子干涉效应就可以从器件中驱动出一个自旋流.适当调节外加偏压、Rashba自旋轨道耦合作用强度以及量子点能级等系统参数时器件中甚至还可以产生完全自旋极化电流或纯自旋流.另外,还讨论了系统参数对自旋流大小、符号、自旋极化度以及峰值的影响.     

Rashba效应对量子线中强耦合束缚极化子性质的影响

采用改进的线性组合算符和幺正变换相结合的方法,研究了Rashba效应对半导体量子线中强耦合束缚极化子性质的影响。计算了Rashba效应的影响下量子线中强耦合束缚极化子的基态能量、有效质量和振动频率。数值分析结果表明:基态能量随库仑束缚势的增加而减少,随受限强度的增加而增大,而且在Rashba效应影响下,基态能量和有效质量比随库仑束缚势的变化曲线由原来的一条分裂为上下两条;振动频率随束缚势、受限强度和耦合强度的增加而加快;有效质量比随库仑束缚势和受限强度的增强而变大。     

基于超快电子自旋动力学的太赫兹辐射研究进展

回顾了近年来利用超快自旋动力学过程产生太赫兹(THz)辐射的研究进展。介绍了基于逆自旋霍尔效应和逆Rashba-Edelstein效应的瞬态自旋流-电荷流转换,指出铁磁/非磁性异质结构已被用于设计低成本、高效率的THz辐射源。通过优化膜厚、生长条件、衬底和结构,可进一步提高基于自旋电子学的THz发射器的效率和带宽。简述了THz发射光谱在研究超快自旋泽贝克效应形成动力学中的应用。     

自旋晶体管的最新研究进展

自旋晶体管是指利用电子自旋自由度构建的在结构上类似于传统半导体晶体管的三端自旋器件。对基于自旋劈裂的磁双极型自旋晶体管、基于热电子输运的自旋晶体管和基于Rashba效应的自旋晶体管的最新研究动态进行了评述,并对其发展前景做了展望。     

基于超快热电效应及自旋热电子学效应的THz波产生

飞秒激光泵浦光电导天线、磁性异质结构、电光晶体、空气等会产生太赫兹脉冲,其产生原理主要基于飞秒激发下载流子、电极化的瞬态变化等非热效应。与此同时,飞秒激光泵浦物质不可避免地会产生超快热效应。近年来,基于超快热电效应以及与自旋相关的超快自旋热电子学效应产生太赫兹波获得越来越多的关注。本文详细介绍了利用塞贝克效应/能斯特效应这两种热电效应,以及自旋塞贝克效应/反常能斯特效应这两种自旋热电子学效应产生太赫兹波的研究进展。超快热电效应及超快自旋热电子学效应已在太赫兹产生方面展现出巨大潜力,有望推动太赫兹源及相关技术的发展。     

Enhanced Spin Seebeck Effect in Monolayer Tungsten Diselenide Due to Strong Spin Current Injection at Interface

The spin current is significantly limited by the spin‐orbit interaction strength, material quality, and spin‐mixing conductance at material interfaces. Such limitations lead to spin current decay at the interfaces, which severely hinders potential applications in spin‐current‐generating thermoelectric devices. Thus, methodical studies on the enhancement of spin currents are indispensable. Herein, a novel approach for enhancing the spin current injected into a normal metal, Pt, using interface effects with a ferromagnetic insulator, yttrium iron garnet (YIG), is demonstrated. This is accomplished by inserting atomically thin monolayer (ML), tungsten diselenide (WSe₂) between Pt and YIG layers. A comparative study of longitudinal spin Seebeck effect (LSSE) measurements is conducted. Two types of ML WSe₂ (continuous and large‐area ML WSe₂ and isolated ML WSe₂ flakes) are used as intermediate layers on YIG film. Notably, the insertion of ML WSe₂ between the Pt and YIG layers significantly enhances the thermopower, V_LSSE/ΔT by a factor of approximately 5.6 compared with that of the Pt/YIG reference sample. This enhancement in the measured LSSE voltages in the Pt/ML WSe₂/YIG trilayer can be explained by the increased spin‐to‐charge conversion at the interface owing to the large spin‐orbit coupling and improved spin mixing conductance with the ML WSe₂ intermediate layer.     

Al0.5Ga0.5N/GaN/Al0.5Ga0.5N量子阱中依赖于表面载流子浓度的Rashba自旋劈裂

通过自洽求解薛定谔和泊松方程,计算了不同表面载流子浓度的Al0.5Ga0.5N/GaN/Al0.5Ga0.5N 量子阱中第一子带的Rashba系数和Rashba自旋劈裂。第一子带在费米能级处的Rashba自旋劈裂可观并且随Ns明显增加,因为Rashba系数特别是费米波矢增加很快。随着Ns的增加,第一子带波函数的峰朝着左异质结界面移动,且阱层的平均电场增加,所以来自阱层和异质结界面的这两个主要贡献部分增加。因而,III族氮化物异质结构中的强极化电场和高浓度的二维电子气对α至关重要,使AlGaN/GaN量子阱的Rashba自旋劈裂同窄带隙的III-V族材料可比。结果表明Ns是影响AlGaN/GaN量子阱中的Rashba系数和Rashba自旋劈裂的一个重要参数,表明这种材料可以应用到自旋电子学器件中。     

Rashba效应下量子点中束缚极化子的有效质量

采用线性组合算符、LLP幺正变换和变分方法,研究Rashba效应下量子点中强耦合束缚极化子有效质量的性质。对RbCl量子点数值计算的结果表明：极化子的振动频率、有效质量均随极化子速率、受限强度和库仑束缚势的增大而增大,原因是极化子速率增大电子动能增加,库仑束缚势增大导致电子-声子间相互作用增强,受限强度增大使电子运动的有效范围减小导致电子能量增大,进而使极化子的振动频率和有效质量增大。受Rashba自旋轨道相互作用的影响,极化子的有效质量发生劈裂,劈裂间距随极化子速率的增大而减小。     

Concepts of Spin Seebeck Effect in Ferromagnetic Metals

Spin Seebeck effect (SSE) and related spin caloritronics have attracted great interest recently. However, the definition of the SSE coefficient remains to be established, let alone a clean experiment to measure the SSE coefficient in ferromagnetic metals. The concept through a model based on the semi‐classical Botlzmann transport equation has been clarified. The model includes the vital spin‐flip process, which is frequent in metals, and points out that the length scale of SSE is much larger than the spin diffusion length. The model reveals how the spin‐flip process influences the transport equations and provides the simple relationship between the different spin‐flip relaxation times for spin‐up and ‐down electrons, which is very useful to understand the spin transport properties. This understanding allows to redefine the expression of the spin Seebeck coefficient.     

Utilization of the Antiferromagnetic IrMn Electrode in Spin Thermoelectric Devices and Their Beneficial Hybrid for Thermopiles

The thermoelectric effect in various magnetic systems, in which electric voltage is generated by a spin current, has attracted much interest owing to its potential applications in energy harvesting, but its power generation capability has to be improved further for actual applications. In this study, the first instance of the formation of a spin thermopile via a simplified and straightforward method which utilizes two distinct characteristics of antiferromagnetic IrMn is reported: the inverse spin Hall effect and the exchange bias. The former allows the IrMn efficiently to convert the thermally induced spin current into a measurable voltage, and the latter can be used to control the spin direction of adjacent ferromagnetic materials. It is observed that a thermoelectric signal is successfully amplified in spin thermopiles with exchange‐biased IrMn/CoFeB structures, where an alternating magnetic alignment is formed using the IrMn thickness dependence of the exchange bias. The scalable signal on a number of thermopiles allowing a large‐area application paves the way toward the development of practical spin thermoelectric devices. A detailed model analysis is also provided for a quantitative understanding of the thermoelectric voltages, which consist of the spin Seebeck and anomalous Nernst contributions.     

Enhancement of Spin–Orbit Interaction by Bandgap Engineering in InAs-Based Heterostructures

We investigated Rashba spin–orbit interaction in various InAs-based heterostructures to evaluate the relative significance of the electric field in the quantum wells (QWs) and at the interfaces. Test structures were designed in such a way that the peak of the electron wave function was located on the abrupt band discontinuity at the front end of the main channel, whereas a control sample had no band discontinuity in the middle of the QW. The Rashba coefficient obtained for the test structures was almost double that of the control sample. Significant contribution of the electric field at the band discontinuity was verified by k · p calculation. Bandgap engineering was shown to be effective for obtaining an increased Rashba coefficient.