1.College of Physics Science and Technology, Bohai University, Jinzhou 121013, China 2.State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China 3.College of Elementary Education, Capital Normal University, Beijing 100048, China 4.Middle School Affiliated to China University of Geosciences, Beijing 100083, China 5.Department of Physics, Capital Normal University, Beijing 100048, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11604021), the Liaoning Revitalization Talents Program, China (Grant No. XLYC20), the Guiding Project of Natural Science Foundation of Liaoning Province, China (Grant No. 2019-ZD-0501), the Science and Technology Research Foundation of Education Commission of Liaoning Province, China (Grant No. LQ2019015), the Science Technology Foundation from Education Commission of Beijing, China (Grant No. KM201810028022), and the Open Project of State Key Laboratory of Low-Dimensional Quantum Physics, China (Grant No. KF201910)
Received Date:26 January 2021
Accepted Date:16 February 2021
Available Online:07 July 2021
Published Online:20 July 2021
Abstract:The major challenge of spintronics lies in how to generate, manipulate, and detect spin current. Multiple methods, such as using magnetic materials, magnetic field, and polarized light field to manipulate the spin of electrons, have been proposed. Owing to the possible applications in spintronic devices, there is currently great interest in the field of spin caloritronics, which focuses on the interplay of spin and heat currents. Stanene is a type of two-dimensional topological insulator consisting of a single layer of Sn atoms arranged in a hexagonal lattice. In this paper, the effects of light and electric fields on the spin-dependent thermoelectric effect of the stanene nanoribbon are studied theoretically based on the non-equilibrium Green’s function method. The results show that the properties and intensity of the thermoelectric current can be effectively controlled by the intensity and the polarization direction of the circularly polarized light field. Under the joint action of a strong circularly-polarized light field and an electric field, the stanene can transform from a quantum spin-Hall insulator into a spin-polarized quantum Hall insulator. When the left-circularly-polarized light field is applied, the spin-down edge states of stanene undergo a phase transition to form a bandgap, and a 100% spin-polarized spin-down current driven by temperature gradient can be obtained. When the right-circularly-polarized light is applied, the edge states of spin-up electrons are destroyed, and a completely polarized spin-up thermal current can be generated. In the weak external field, the properties of the edge state do not change, and the system does not output a thermoelectric current. In addition, the study shows that the intensity of the thermal spin current is related to the width of the bandgap, and a moderate increase in temperature can significantly increase the peak value of the current, but the higher equilibrium temperature and temperature gradient will restrain the spin thermoelectric effect. Keywords:spin dependent thermoelectric effect/ circularly-polarized light field/ stanene
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--> --> --> 1.引 言IV主族元素单原子层结构的理论研究最早可以追溯到1994年, Takeda和Shiraishi[1]基于密度泛函理论研究指出IV主族元素可以形成单原子层结构, 并给出了相应的原子和能带结构. 由于当时科学界普遍认为有限温度下热力学稳定的二维晶体并不存在, 这一理论成果并未引起科研人员的广泛兴趣. 直到2004年, Novoselov等[2]利用机械剥离方法成功从石墨中分离出石墨烯, 长程有序的二维纳米材料重新受到科研人员的关注, 逐渐有更多的科研人员开始投入到石墨烯以外的IV主族单原子层材料的研究. 2009年, Sahin等[3]利用第一性原理计算表明, 与硅烯和锗烯类似, 锡原子也可形成稳定的低翘曲蜂窝状二维结构. 相比于硅烯和锗烯, 锡烯具有更强的自旋轨道耦合. 2013年, Xu等[4]的理论研究表明, 锡烯的体能隙要远大于硅烯和锗烯, 能够达到100 meV, 并且具有优越的热电效应[5]. 2015年以来, 国内外多个研究组相继在不同的衬底上成功实现了翘曲和平面结构锡烯的制备[6-11]. 热电效应利用材料实现热能与电能直接相互转换, 其中利用温度差产生电流或电荷积累的现象, 被称为塞贝克(Seebeck)效应. 2010 年, Slachter等[12]在三端自旋阀结构中观测到了因自旋通道彼此独立而产生的自旋依赖塞贝克效应(spin-dependent Seebeck effect). 自旋热电效应把电子和空穴的输运特性与电子的自旋特性相结合, 从热学方面为自旋流的产生和操控提供了新的途径, 拓展了自旋电子学的研究空间[13,14]. 理论和实验研究表明, 低维材料中的热电效应比三维块体材料更为显著[15-18]. 伴随着体系维度降低所带来的新奇特性, 二维纳米材料逐渐受到科研人员的关注, 理论上不断有新材料被提出和研究[19,20]. 近些年, 基于IV主族拓扑绝缘体(硅烯、锗烯、锡烯), 一系列理论和实验工作对电场和铁磁交换场调控的自旋热电效应展开了研究[21-33]. 本文从理论上研究圆偏振光场和电场对锡烯自旋热输运性质的影响. 如图1所示, 考虑对锡烯纳米带的左右两个热极施加不同温度的热源, 在中心器件区域通过背电极施加Z轴方向的电场, 并对锡烯的上表面辐照圆偏振光场. 本文重点讨论偏振光场的极化方向和强度、电场的方向和强度以及系统温度和温差对自旋热电流的影响. 图 1 (a) 施加温度差的锡烯纳米带俯视图. 红色和蓝色区域表示高温和低温热极, 热极的温度分别为$T_{\rm L} = T+ $$ \Delta T/2$和$ T_{\rm R} = T-\Delta T/2 $, 灰色区域表示圆偏振光场辐照的区域. (b) 施加圆偏振光场和电场的锡烯纳米带俯视图, 中间灰色区域的背电极为锡烯提供Z轴方向的电场 Figure1. (a) Top view of a stanene nanoribbon with temperature difference. The red and blue regions represent the high-temperature and low-temperature leads. The temperatures of the thermal leads are $ T_{\rm L} = T+\Delta T/2 $ and $ T_{\rm R} = T-\Delta T/2 $, respectively. The gray central region represents the area irradiated by the circularly polarized light field. (b) Top view of the stanene nanoribbon with circularly polarized light and electric fields, the back gate in the gray area provides the electric field in the Z-axis direction.