四端双量子点系统中的自旋和电荷能斯特效应

Spin and charge Nernst effect in a four-terminal double quantum dot system

基于非平衡态格林函数方法,理论研究了与四个电极耦合的双量子点系统中的自旋和电荷能斯特效应,考虑了不同电极的磁动量结构和量子点内以及量子点间电子的库仑相互作用对热电效应的影响.结果表明铁磁端口中的磁化方向能够有效地调节能斯特效应:当电极1和电极3中的磁化方向反平行排列时,通过施加横向的温度梯度,系统中将会出现纯的自旋能斯特效应;当电极4从普通金属端口转变为铁磁金属端口时,将同时观测到电荷和自旋能斯特效应.研究发现,能斯特效应对于铁磁电极极化强度的依赖程度较弱,但对库仑排斥作用十分敏感.在量子点内和点间库仑排斥作用的影响下,自旋及电荷能斯特系数有望提高两个数量级.

With the increase of integration scale, heat dissipation becomes one of the major problems in high density electronic devices and circuits. Controlling and reusing the heat energy in such miniaturized structures are essential topics for current and future technologies. With the development of microfabrication technology and low-temperature measurement technology in the last two decades, the thermoelectric measurement in low-dimensional sample has been feasible, and the thermal transport has received more and more attention. For the multi-terminal device, there is a novel thermoelectric phenomenon, called the spin Nernst effect, in which spin currents （or spin voltages） are generated perpendicularly to the temperature gradient. The spin Nernst effect has been confirmed experimentally, and has been theoretically studied in a variety of materials. In this paper, the spin and charge Nernst effect in a pair of vertically aligned quantum dots attached to four leads are studied in the Coulomb blockade regime based on the nonequilibrium Green＇s function technique. We focus on the influences of magnetic configuration and intra-dot （inter-dot） Coulomb interaction on the spin and charge Nernst effect. It is found that the signs and the magnitudes of spin and charge Nernst effect can be modulated by adjusting the magnetization directions of ferromagnetic electrodes. When the magnetic moments in the 1 and 3 electrodes are turned to antiparallel alignment, the pure spin Nernst （without charge Nernst） effect can occur by applying a transverse temperature gradient. Conversely, the spin and charge Nernst effect disappear if the magnetic moments of lead 1 and lead 3 are in the case of parallel configuration. Except for left and right thermal leads, we investigate the effect of the middle lead （lead 4） on the property of the Nernst effect. We find that when the normal metal lead 4 is transferred to ferromagnetic metal, the spin and charge Nernst effect both can be obtained simultaneously. In the end of the paper, we study the influences of intra-dot and inter-dot Coulomb interaction on the spin dependent Nernst coefficient. Through numerical calculations, we demonstrate that the magnitude of the Nernst effect is less dependent on the polarization strength of ferromagnetic electrodes, but can be remarkably enhanced by the Coulomb blockade. The spin Nernst coefficient is predicted to be more than two orders of magnitude larger than that of the case of zero Coulomb interaction. All the results indicate that the proposed four-terminal double quantum dot nano system is a promising candidate for spin caloritronic device.