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利用非平衡格林函数方法理论研究了光场和电场对锡烯纳米带自旋相关热电效应的影响. 研究表明, 热电电流的性质和强度可以通过圆偏振光场的强度和偏振化方向进行有效调控. 在较强的左旋圆偏振光场和电场的共同作用下, 锡烯自旋向下的边缘态发生相变形成带隙, 通过温度梯度的驱动可以获得100%极化的自旋向下的自旋流. 当施加右旋偏振光时, 自旋向上的边缘态被破坏, 可以产生完全极化的自旋向上的热自旋流. 在较弱的外场作用下, 边缘态的性质不发生改变, 系统不对外输出热电电流. 此外, 研究表明热自旋流的大小与带隙的宽度有关, 适度地增加温度可以显著增大热自旋流的峰值, 但是较高的平衡温度和温度梯度将抑制自旋热电效应.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.
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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Fig. 1. (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.
图 2 (a) 电场交错势能
$ \lambda_E $ 和偏振光场强度参数$ \lambda_\varOmega $ 分别取$ \lambda_E = \lambda_\varOmega = 0 $ , 0.02, 0.04, 0.06, 0.08, 0.10 eV时, 自旋相关的电流$ I_\sigma $ 随左(右)热极费米能级$ E_{\rm F} $ 的变化; (b)$ \lambda_E = \lambda_\varOmega = 0.04 $ eV和(c)$ \lambda_E = \lambda_\varOmega = 0.08 $ eV时的电子能带结构, 其中红色虚线代表自旋向上电子形成的能带, 蓝色实线对应自旋向下的能带Fig. 2. (a) Spin dependent current
$ I_\sigma $ as a function of the Fermi energy$ E_{\rm F} $ with different values of electric-field-induced staggered potential and light parameter$ \lambda_E = \lambda_\varOmega = 0 $ , 0.02, 0.04, 0.06, 0.08, and 0.10 eV. Energy-band diagrams of stanene with different values of$ \lambda_E $ and$ \lambda_\varOmega $ : (b)$ \lambda_E = \lambda_\varOmega = 0.04 $ eV; (c)$ \lambda_E = \lambda_\varOmega = 0.08 $ eV. The red dash and blue solid lines represent spin-up and spin-down energy states, respectively.
图 3 (a) 电场交错势能
$ \lambda_E = 0.1 $ eV, 圆偏振光场参数分别取$ \lambda_\varOmega = -0.025 $ , –0.050, –0.075, –0.100 eV时自旋相关电流$ I_\sigma $ 随电极费米能级$ E_{\rm F} $ 的变化关系; (b)$ \lambda_E = 0.1 $ eV且$\lambda_\varOmega = -0.050$ eV时锡烯的电子能带结构, 其中红色虚线代表自旋向上电子形成的能带, 蓝色实线对应自旋向下的能带Fig. 3. (a) Spin dependent current
$ I_\sigma $ as a function of the Fermi energy$ E_{\rm F} $ with$ \lambda_E = 0.1 $ eV and$ \lambda_\varOmega = -0.025 $ , –0.050, –0.075, –0.100 eV; (b) energy-band diagram of stanene with$ \lambda_E = 0.1 $ eV and$\lambda_\varOmega = -0.050$ eV. The red dash and blue solid lines represent spin-up and spin-down energy states, respectively.
图 4 (a) 热极费米能级
$ E_{\rm F} = 0.05 $ eV时, 总的热电电流$ I_{\rm e} $ 随着圆偏振光场参数$ \lambda_\varOmega $ 和电场交错势能$ \lambda_E $ 的变化; (b) 系统温度$ T = 100 $ K、温度梯度$ \Delta T $ 取不同值时热自旋相关电流$ I_\sigma $ 随着热极费米能级$ E_{\rm F} $ 的变化; (c) 温度梯度$ \Delta T = 50 $ K、系统温度$ T = 200 $ , 300, 400 K时总的热电电流$ I_{\rm e} $ 随着热极费米能级的变化. 图(b)和图(c)中光场强度参数和电场交错势能分别为$ \lambda_\varOmega = -0.05 $ eV和$ \lambda_E = 0.1 $ eVFig. 4. (a) Total thermoelectric current
$ I_{\rm e} $ as a function of electric-field-induced staggered potential$ \lambda_E $ and light parameter$ \lambda_\varOmega $ with the Fermi energy of thermal electrode$ E_{\rm F} = 0.05 $ eV; (b) spin dependent current$ I_\sigma $ versus Fermi energy$ E_{\rm F} $ with system equilibrium temperature$ T = 100 $ K and various temperature gradient$ \Delta T $ ; (c) total thermoelectric current$ I_{\rm e} $ as a function of Fermi energy$ E_{\rm F} $ with system equilibrium temperature$T = 200$ , 300, and 400 K and$ \Delta T = 50 $ K. The electric-field-induced staggered potential and light parameter in panel (b) and panel (c) are$ \lambda_\varOmega = -0.05 $ eV and$ \lambda_E = 0.1 $ eV, respectively. -
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Google Scholar
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