金属薄膜中的逆自旋霍尔效应

自旋流的产生和测量是自旋电子学面临的重大挑战．逆自旋霍尔效应提供了对自旋流进行电学测量的有效手段．文章总结了近年来人们对金属薄膜中的逆自旋霍尔效应的研究,从非局域电注入、铁磁共振注入、声波共振注入和圆偏振光注入这四种不同的自旋流注入方式来介绍逆自旋霍尔效应的物理机制、实现方式和影响因素．     

NiFe/Pt薄膜中角度相关的逆自旋霍尔效应

NiFe/Pt双层薄膜样品在铁磁共振时, NiFe磁矩进动所产生的自旋流注入到Pt层中, 由于逆自旋霍尔效应产生直流电压V_ISHE, 此电压会叠加到NiFe薄膜由于自旋整流效应而产生的电压V_SRE 上, 实验测量所得电压为V_ISHE和V_SRE的叠加. 为了区分这两种不同机理对电压的贡献, 本文采取旋转外加静磁场的方法, 通过分析所测电压随磁场角度的变化从而分离出V_ISHE 的大小. 研究结果表明, 相比于单层NiFe(20 nm)薄膜样品, NiFe(20 nm)/Pt(10 nm)双层膜样品中由于NiFe自旋注入到Pt 中导致铁磁共振线宽增加. 与逆自旋霍尔效应产生的电压相比, 自旋整流效应的贡献较小, 但不可忽略. 本文工作有助于认清铁磁/非磁性金属材料中的自旋相关效应, 并提供了一种准确的分析逆自旋霍尔效应的方法.     

Inverse Spin Hall Effect in Two-Terminal Device with Rashba Spin-Orbit Coupling

We report a theoretic study on the inverse spin-Hall effect （ISHE） in a two-terminal nano-device that consists of a two-dimensional electron gas （2DEG） with Rashba spin-orbit coupling （RSOC） and two ideal leads. Based on a two-site toy model and Keldysh Green＇s function method, we derive an analytic result of ISHE, which shows clearly that a nonzero transverse charge current stems from the combined effect of the RSOC, the spin bias, and its spin polarization direction in spin space. Our further numerical calculations in a larger system other than two-site lattice model demonstrate that the transverse charge current, dependent on the strength of the RSOC, the Fermi energy of the system, as well as the system size, can exhibit oscillating behavior and even reverse its sign due to Rashba spin precession. These properties may be helpful for eficient detection of the spin current （spin bias） by measuring the transverse charge current in a spin-orbital coupling system.     

Spin Hall Magnetoresistance and Spin Nernst Magnetothermopower in a Rashba System: Role of the Inverse Spin Galvanic Effect

In ferromagnet/normal‐metal bilayers, the sensitivity of the spin Hall magnetoresistance and the spin Nernst magnetothermopower to the boundary conditions at the interface is of central importance. In general, such boundary conditions can be substantially affected by current‐induced spin polarizations. In order to quantify the role of the latter, we consider a Rashba two‐dimensional electron gas with a ferromagnet attached to one side of the system. The geometry of such a system maximizes the effect of current‐induced spin polarization on the boundary conditions, and the spin Hall magnetoresistance is shown to acquire a non‐trivial and asymmetric dependence on the magnetization direction of the ferromagnet.     

铁磁石墨烯体系的CT不变量子自旋霍尔效应

文章作者在垂直磁场作用下的铁磁石墨烯体系里预言了一种新类型的量子自旋霍尔效应.这量子自旋霍尔效应与自旋轨道耦合无关,体系也不具有时间反演不变性;但是有CT不变（C为电子-空穴变换、T为时间反演变换）.由于量子自旋霍尔效应,体系的纵向电阻和自旋霍尔阻出现量子化平台.特别是,自旋霍尔阻的量子化平台有很强的抗杂质干扰能力.     

Quantum spin Hall effect in a square-lattice model under a uniform magnetic field

We study a toy square-lattice model under a uniform magnetic field. Using the Landauer-Bttiker formula, we calculate the transport properties of the system on a two-terminal, a four-terminal and a six-terminal device. We find that the quantum spin Hall (QSH) effect appears in energy ranges where the spin-up and spin-down subsystems have different filling factors. We also study the robustness of the resulting QSH effect and find that it is robust when the Fermi levels of both spin subsystems are far away from the energy plateaus but is fragile when the Fermi level of any spin subsystem is near the energy plateaus. These results provide an example of the QSH effect with a physical origin other than time-reversal (TR) preserving spin-orbit coupling (SOC).     

Spin Chern numbers and time-reversal-symmetry-broken quantum spin Hall effect

The quantum spin Hall （QSH） effect is considered to be unstable to perturbations violating the time-reversal （TR） symmetry. We review some recent developments in the search of the QSH effect in the absence of the TR symmetry. The possibility to realize a robust QSH effect by artificial removal of the TR symmetry of the edge states is explored. As a useful tool to characterize topological phases without the TR symmetry, the spin-Chern number theory is introduced.     

掺铌SrTiO_3中的逆自旋霍尔效应

采用磁控溅射法在未掺杂和掺杂的SrTiO_3基片上沉积了NiFe薄膜,通过翻转测试法分离出掺杂样品中的自旋整流电压和逆自旋霍尔电压.研究结果表明:在未掺杂的SrTiO_3基片中,翻转前后测试的电压曲线基本一致,为NiFe薄膜自旋整流效应产生的电压.对于掺Nb浓度x为0.028, 0.05, 0.1, 0.15, 0.2的SrTiO_3基片,分离出的逆自旋霍尔电压随掺杂浓度增加而减小,在掺杂浓度为0.15和0.2的样品中没有探测到明显的逆自旋霍尔电压.本文的结果表明,在SrTiO_3中掺入强自旋轨道耦合的杂质,通过掺杂浓度可以实现对SrTiO_3中逆自旋霍尔效应的调控,这类可调控的自旋相关研究为自旋电子器件的研究和开发提供了更多的可能性,具有很大的潜在应用价值.     

Quantum anomalous Hall effect in real materials

Under a strong magnetic field,the quantum Hall(QH) effect can be observed in two-dimensional electronic gas systems.If the quantized Hall conductivity is acquired in a system without the need of an external magnetic field,then it will give rise to a new quantum state,the quantum anomalous Hall(QAH) state.The QAH state is a novel quantum state that is insulating in the bulk but exhibits unique conducting edge states topologically protected from backscattering and holds great potential for applications in low-power-consumption electronics.The realization of the QAH effect in real materials is of great significance.In this paper,we systematically review the theoretical proposals that have been brought forward to realize the QAH effect in various real material systems or structures,including magnetically doped topological insulators,graphene-based systems,silicene-based systems,two-dimensional organometallic frameworks,quantum wells,and functionalized Sb(111) monolayers,etc.Our paper can help our readers to quickly grasp the recent developments in this field.     

Possible Topological Hall Effect in a Spin Gapless Semiconducting Mn2CoAl Heusler Compound

Topological Hall effect (THE), induced by the interaction of charge carriers with mesoscopic or microscopic noncoplanar spin structures, holds promising applications in the field of spintronics. In the present study, a giant THE of about 2 μΩ cm at room temperature is reported in a bulk spin gapless semiconducting Mn₂CoAl cubic Heusler compound. Temperature-dependent investigation of magneto-transport data reveals that the Mn₂CoAl has the large THE over a wide temperature range of 2–300 K. The alternating current (AC) susceptibility as a function of magnetic field exhibits a smooth and continuous response rather than any kink or anomaly, suggests that the observed THE in the Mn₂CoAl compound results from the interaction of charge carriers with the microscopic noncoplanar spin texture. The observed THE as a function of temperature follows the same behavior as the magnetocrystalline anisotropy (MCA) of the cubic Mn₂CoAl, indicating the competition of the MCA with ferromagnetic and antiferromagnetic exchange interactions as the origin of the noncoplanar spin texture and hence THE. Micromagnetic simulations further support the emergence of noncoplanar spin structure as a result of the competition between different energies.     

Quantum spin Hall and quantum valley Hall effects in trilayer graphene and their topological structures

The present study pertains to the trilayer graphene in the presence of spin orbit coupling to probe the quantum spin/valley Hall effect. The spin Chern-number C_s for energy-bands of trilayer graphene having the essence of intrinsic spin–orbit coupling is analytically calculated. We find that for each valley and spin, C_s is three times larger in trilayer graphene as compared to single layer graphene. Since the spin Chern-number corresponds to the number of edge states,consequently the trilayer graphene has edge states, three times more in comparison to single layer graphene. We also study the trilayer graphene in the presence of both electric-field and intrinsic spin–orbit coupling and investigate that the trilayer graphene goes through a phase transition from a quantum spin Hall state to a quantum valley Hall state when the strength of the electric field exceeds the intrinsic spin coupling strength. The robustness of the associated topological bulk-state of the trilayer graphene is evaluated by adding various perturbations such as Rashba spin–orbit(RSO) interaction αR, and exchange-magnetization M. In addition, we consider a theoretical model, where only one of the outer layers in trilayer graphene has the essence of intrinsic spin–orbit coupling, while the other two layers have zero intrinsic spin–orbit coupling.Although the first Chern number is non-zero for individual valleys of trilayer graphene in this model, however, we find that the system cannot be regarded as a topological insulator because the system as a whole is not gaped.     

转换反射中光自旋霍尔效应的自旋堆积方向

从理论上和实验上研究了转换反射中光自旋霍尔效应的自旋堆积方向的方法,建立了描述光束在空气-棱镜界面反射的自旋堆积模型,揭示了横移与光束入射偏振角的定性关系。研究发现,当入射角小于布儒斯特角时,随着入射偏振角的逐渐增大,自旋堆积的方向发生反转。而当入射角大于布儒斯特角时,自旋堆积的方向不再随入射偏振角的变化而反转。结果表明,在光束入射角为确定值且小于布儒斯特角的情况下,可以通过调控光束的入射偏振角转换自旋堆积的方向。转换自旋堆积方向的研究为有效调控光自旋霍尔效应提供了新的途径。     

The origin of spin current in YIG/nonmagnetic metal multilayers at ferromagnetic resonance

Spin pumping in yttrium-iron-garnet(YIG)/nonmagnetic-metal(NM) layer systems under ferromagnetic resonance(FMR) conditions is a popular method of generating spin current in the NM layer.A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications.It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer.Here,by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM_1/NM_2(NM_1 =Cu or Al,NM_2 =Pt or Ta),we unambiguously showed that spin current in NM,instead of from the precessing YIG magnetization,came from the magnetized NM surface(in contact with thin YIG),either due to the magnetic proximity effect(MPE) or from the inevitable diffused Fe ions from YIG to NM.This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect(ISHE).The voltage signal is attributed to the magnetized NM surface,hardly observed in the conventional FMR experiments,and was greatly amplified when the electrical detection circuit was switched on.     

柱矢量光束在透射时的自旋霍尔效应

有限光束矢量特性可以由全局化的特征单位矢量和广义琼斯矢量完整地描述,特征单位矢量平行于传播轴的光束为柱矢量光束。通过这种表示理论描述了在近轴近似条件下柱矢量光束的透射情况。在透射过程中光束的特征单位矢量发生了变化,透射光束的特征单位矢量既不平行于传播轴,也不垂直于传播轴。特征单位矢量的变化导致光束产生了一个与入射面垂直的横向位移,同时这个横向位移和光束的偏振椭圆率有关,不同圆偏振态(左旋和右旋)的透射光束所产生的横向位移的方向相反。这一结果和近年来在实验中检测到的均匀偏振光束的自旋霍尔效应类似,表明柱矢量光束在透射过程后也会产生光的自旋霍尔效应。