Quantum spin Hall effect spintronics

在这个项目中，我们将调查潜在的自旋电子学的量子自旋霍尔（QSH）制度的混合纳米结构，通过连接铁磁金属接触的边缘状态的二维拓扑绝缘体。这些2D材料将由半导体InAs/GaSb耦合量子威尔斯形成。能够利用自旋动量锁定的螺旋边缘状态的QSH制度将有可能实现显着降低功耗的经典ICT硬件，并在长期提供的前景是有用的拓扑量子计算。为了制造这样的自旋电子器件，我们需要知道电流流过其边缘态的条件。我们需要知道从铁磁体注入到QSH边缘状态的电流的自旋极化，以及哪种铁磁体接触材料提供最大的自旋极化。我们需要知道如何有效地自旋可以注入和检测这些QSH边缘通道使用铁磁金属接触。我们还需要知道自旋信息可以在QSH边缘状态中传播的距离，以及在什么情况下这个距离是最长的。该项目是物理和天文学院与电子和电气工程学院的合作，该学院在自旋电子学和铁磁材料以及拓扑材料的设备研究方面具有专业知识，他们能够在III-V族半导体分子束外延系统中生长超高质量的InAs/GaSb耦合量子威尔斯阱。我们将开始通过构建接触的InAs/GaSb台面与顶部和底部的栅极，使他们能够被调整到一个电荷中性和非平凡的制度，这是正确的条件，电流流动，只有在边缘状态。我们将在InAs/GaSb梅萨上的铁磁源极接触的两侧连接正常的漏极接触，并在非平凡边缘态区域中测量来自左流动边缘态和右流动边缘态的漏极电流;在QSH边缘状态中的自旋动量锁定将意味着这些空间分离的电流直接对应于自旋分辨电流，允许直接测量从铁磁体注入的电流的自旋极化。我们将尝试不同的铁磁性金属，以确定哪一种效果最好。然后，我们将研究两个紧密间隔的铁磁接触之间的QSH边缘状态中的电流的流动，当电流流动方向被自旋动量锁定到接触的多数自旋方向时，电流的流动预计会更大;反转接触中的磁化方向将反转这种二极管状行为。正向和反向电流之间的差异将告诉我们自旋注入和检测的效率。通过测量正向电流和反向电流之间的差值随间距的减小，我们可以将触点分开，从而确定自旋在边缘态内相干流动的长度;我们将研究它作为温度的函数，以确定导致自旋相干损失的物理机制。我们将获得的结果不仅将通过阐明这种新型材料组合提供的可能性来产生高影响力的出版物和会议演讲，而且还将在量子自旋霍尔自旋电子学领域开发有价值的技术应用。

In this project we shall investigate the potential for spintronics of the quantum spin Hall (QSH) regime in hybrid nanostructures made by attaching ferromagnetic metal contacts to the edge states of two-dimensional topological insulators. These 2D materials will be formed from semiconducting InAs/GaSb coupled quantum wells. Being able to harness the spin-momentum-locked helical edge states in the QSH regime will have the potential for realising dramatic reductions in the power consumption of classical ICT hardware, and in the longer term offer the prospect of being useful for topological quantum computing. To build such spintronic devices, we need to know the conditions under which current flows through their edge states. We need to know the spin polarisation of a current injected from a ferromagnet into the QSH edge state, and which ferromagnetic contact material provides the largest spin-polarisation. We need to know how efficiently spins can be injected and detected in these QSH edge channels using ferromagnetic metal contacts. We also need to know over what distance spin information can propagate in the QSH edge states, and in what circumstances this distance is the longest.The project is a collaboration between the School of Physics and Astronomy, who have expertise in spintronics and the study of devices incorporating ferromagnetic materials, as well as topological materials, and the School of Electronic and Electrical Engineering, who are capable of growing ultra-high quality InAs/GaSb coupled quantum wells in their III-V semiconductor molecular beam epitaxy system. We will begin by constructing contacted InAs/GaSb mesas with top and bottom gates that allow them to be tuned into a charge-neutral and non-trivial regime, which are the correct conditions for current to flow only in the edge states. We will attach normal drain contacts on either side of a ferromagnetic source contact on a InAs/GaSb mesa and measure the drain currents from left- and right-flowing edge states in the non-trivial edge state regime; the spin-momentum locking in the QSH edge states will mean that these spatially separated currents directly correspond to the spin-resolved currents, allowing a direct measurement of the spin-polarisation of the current injected from the ferromagnet. We shall try different ferromagnetic metals to determine which one works best. We will then study the flow of a current in a QSH edge state between two closely-spaced ferromagnetic contacts, which is expected to be larger when the current flow direction is spin-momentum locked to the majority spin direction of the contacts; reversing the magnetisation direction in the contacts will invert this diode-like behaviour. The difference between forward and reverse currents will tell us the efficiency of the spin injection and detection. Moving the contacts apart will allow us to determine the length over which spins can flow coherently within the edge states by measuring the decline in difference between forward and reverse currents with spacing; we shall study this as a function of temperature in order to determine the physical mechanisms causing the loss of spin coherence. The results we shall obtain will not only lead to high impact publications and conference presentations by shedding light on the possibilities offered by this novel combination of materials, but also develop valuable know-how in the field of quantum spin Hall spintronics for technological applications.