【作者】 尹海涛；

【导师】 吕天全；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 自旋电子学是一门研究如何在固体中有效地控制自旋自由度的新兴交叉学科,人们希望利用自旋自由度来取代(或者结合)传统电子学器件中的电荷自由度,从而实现新型的自旋电子学器件。研究自旋电子学器件要解决的主要问题有:如何有效地产生自旋极化;自旋极化保持的时间和输运的距离,以及如何操控它们;自旋极化的探测。论文将基于Rashba自旋轨道耦合,采用非平衡格林函数方法和slave-boson平均场近似,研究耦合双量子点体系结构中自旋积累、自旋极化输运、纯自旋流产生以及近藤效应等问题,并讨论量子点能级、偏压、磁通量、自旋轨道耦合等多种因素的影响下,量子点体系的自旋极化输运性质。首先,从理论上研究平行耦合双量子点与电极对称连接的结构中,Rashba自旋轨道耦合作用对Fano共振的影响。研究表明,受Rashba自旋轨道耦合作用调节的共振通道,非共振通道及它们之间的量子相干是Rashba自旋轨道耦合作用调控Fano共振的根本原因。Rashba自旋轨道耦合作用和磁通量在产生自旋极化输运的过程中起到关键性作用。当没有磁通量通过量子点体系时( ? = 0),与自旋无关的Fano共振受Rashba自旋轨道耦合作用强度调节。当有磁通量通过量子点体系时,如? =π/2,自旋相关的Fano共振出现。而且每种自旋成份的Fano共振峰线型可被Rashba自旋轨道耦合作用强度调控。考虑电子间的库仑相互作用后,不同自旋电子的电导谱分为两部分,一部分中心位于成键态和反成键态,另一部分中心位于其库仑阻塞区。其次,通过对每个臂均嵌入一个量子点的Aharonov-Bohm(AB)环中的自旋积累效应的研究,发现在Rashba自旋轨道耦合作用下,即使在无铁磁性电极、无磁场的情况下,由于量子相干,量子点中也会产生自旋积累,并且量子点中的自旋积累有如下特点:(1)自旋积累的大小和方向可通过门电压和偏压调节; (2)每个量子点上的自旋积累作为? R的函数,周期为2π; (3)自旋积累最大值的位置可由量子点的能级、电子间的库仑相互作用强度和偏压等因素共同调节。此外,研究了平行耦合双量子点与三电极连接的体系自旋极化输运性质,利用自旋轨道相互作用的可控性,探讨控制不同自旋的电子沿不同电极输运的可能性,提供产生自旋分离及纯自旋电流的新途径。研究表明当恰当地选取Rashba自旋轨道耦合强度和偏压后,通过调节施加在量子点上的门电压,改变其中一个量子点能级时,一个由中间电极注入的非极化电流,经过量子点体系后,在左、右电极实现自旋的分离,即二个完全极化且极化方向相反的自旋流可分别萃取到左电极或右电极上。另外,在三个电极的电压满足一定关系时,中间电极可获得一个幅度和方向均可调节的纯自旋流。这样的模型结构简单,并可使用现有的技术手段在实验上实现。最后,利用slave-boson平均场近似,研究在Rashba自旋轨道耦合作用影响下,双量子点体系的近藤效应。发现在磁通和Rashba自旋轨道耦合共同作用下,近藤区,量子点体系出现自旋极化的隧穿函数、局域态密度和线性电导。而且如果适当地调节Rashba自旋轨道耦合强度,可控制同一时间只有一种自旋方向电子能通过量子点体系,该性质可用于设计自旋阀。总之,论文基于Rashba自旋轨道相互作用,探索利用电场、偏压等电学手段在量子点体系中实现自旋极化输运、自旋调控的可能性。并明确Rashba自旋轨道相互作用、库仑相互作用及体系的结构参数等可调控因素对自旋极化输运的性质的影响,揭示受自旋轨道相互作用支配或影响的新物理规律。提出实现电学方法操控电子自旋的纳米器件理论模型,为自旋电子学器件设计和量子信息实现提供理论信息。更多还原

【Abstract】 The purpose of spintronics is to manipulate the spin degree of freedom instead of (or together with) the charge degree of freedom in solid-state system. In this multidisciplinary field, people try to develop spintronic devices to substitute the traditional electronic devices. The three of the most important issues in this field are: (1) prolonging the spin decoherence time, (2) efficient generating spin polarization and (3) measurement of the spin polarization.In this dissertation, by means of the non-equilibrium Green’function and the slave boson mean-field approximation, we investigate the spin accumulation, spin-polarizition transport properties and pure spin currents in double quantum dots (DQDs), in which the Rashba spin-orbit(RSO) interaction exists, as well as the Kondo-assisted transport in Kondo regime. We investigate the influences of the quantum dot level, magnetic flux, the bias voltage, and the RSO interaction on the spin polarized transport properties of coupled-quantum-dot system theoretically and get some significant results.Firstly, we investigate the spin-dependent Fano effect through parallel-coupled DQDs connected to two electrodes symmetrically. We found that the base cause of manipulating the Fano line shape in the conductance spectra is that the resonace channel, non-resonace channel, and interference between these two channels can be tuned by RSO interaction. The RSO interaction and the magnetic flux play an essential role in spin-resolved transport through the system in interacting regime. For the case with ? = 0, the spin-independent Fano resonance can be controlled by the strength of the RSO interaction; when the magnetic flux is switched on, the Fano line shape of each spin component can be modulated by the value of ? R. Moreover, the intradot electron-electron interaction is taken into account, the conductance spectra of each spin component is divided into two sections. One centered at the bonding state and antibonding state, the other at their Coulomb blockade counterparts.Secondly, we investigated the spin accumulation of Aharonov-Bohm (AB) interferometer, in which an interacting quantum dot (QD) is inserted in each arm. The spin accumulation in QD emerges in presence of RSO interaction even though no magnetic materials and magnetic field. It is found that the following objectives can be achieved: (1) the magnitude and direction of the spin accumulation in the QD can be controlled and tuned by the gate voltage or the bias; (2) the spin accumulation in two dots is sensitive to the strength of RSO interaction andΔnias function of ? Rexhibits a periodic function with the period of 2π; (3) the position of the maximum of the spin accumulation can be modulated by the intradot level , Coulomb interaction and the bias together.Thirdly, the spin–polarized current through a three-terminal parallel-coupled DQDs device has been investigated by taking account of both the RSO interaction and intradot Coulomb interaction. The spin-dependent current formulas and relevant Green functions are obtained within the framework of the Hartree-Fock approximation. We find that by properly choosing the strength of RSO interaction and bias voltages, the spin-up and spin-down currents can be extracted into the left and right leads, respectively. In particular, a pure spin current, whose the magnitude and direction can be tuned by arranging the bias voltage applied between the three leads, can be driven out from the middle lead.Finally, by means of the salve-boson mean field approximation, we study the transport properties of DQDs in Kondo regime with RSO interaction. We show that the transmission probability, the local density of states, and linear conductance of each spin electron become different by tuning RSO interaction strength with certain magnetic flux. We find that by properly choosing the strength of RSO interaction,one spin component is allowed to pass the structure, but the other spin component is forbidded. This property can be used to designe the spin valve.In conclusion,we systematically investigate the spin polarized transport properties through double quantum dots system, especially emphasis on the influcences of the electronic methods on the manipulation of spin. It is clarified how the Rashba spin-orbit interaction, Coulomb interaction and structure parameters change the spin polarized transport properties. The results, we obtained, can be applied to design of spintronics devices and quantum computation.更多还原