Floquet拓扑绝缘体输运性质的理论研究

In recent years, topological phases of matter have been intensively studied in condensed matter physics, and it has been appreciated that Floquet topological insulators can be produced in periodically driven systems. Floquet topological insulators expand the family of materials with desirable topological characteristics and at the same time create new physical phenomena. However, the transport properties of edge-channel transport in topological insulators and the detection of Majorana Fermions are currently important and still open issues. In this project, we will use tight-binding model to describe the electronic structure of materials, and the transport properties in nanostructures are studied using scattering wave functions and Green's functions. We will provide guidelines for the developments of topological quantum devices and the detection of Majorana Fermions. The major efforts of this project will be addressed as follows: 1) We will explore other Floquet topological insulators in various promising candidate materials, and it is of great interest to study the physical properties of different material systems so as to understand how the topological characteristics can be modified. 2) Transport properties of Floquet topological insulators will be studied in various nanostructures, we will propose possible topological quantum devices based on our findings.The effects of the robustness of edge states on transport properties will be explored, and we will also study the transport properties under the quantum confinement effects. 3) Floquet Majorana Fermions are realized in a hybrid system with a Floquet topological insulator proximity coupled to a superconductor. We study the transport properties of Floquet Majorana Fermions, and how the Floquet Majorana Fermions can be controlled and detected.

物质的拓扑相是近年来凝聚态物理领域的研究热点，而通过周期性外场与物质作用来实现Floquet拓扑绝缘体已成为扩展拓扑材料范围、发现新拓扑性质的重要的研究方向。然而，拓扑边缘态的输运性质、马约拉纳费米子的实验室观测目前仍然是该领域具有挑战性的重要课题。本项目将从紧束缚模型出发，利用散射波函数和格林函数方法，来探索Floquet拓扑绝缘体电子和自旋的输运性质，为拓扑量子器件的发展和马约拉纳费米子的实验观测提供指导思路。主要内容包括：1）在一些理想候选材料中预言新的Floquet拓扑绝缘体，揭示其物理性质和拓扑态调控方法。2）考虑Floquet拓扑绝缘体不同纳米结构的输运性质，研究量子限域效应、边缘态鲁棒性等对纳米结构输运性质的影响，发展Floquet拓扑量子器件。3）在Floquet拓扑绝缘体中引入超导体作用，深入研究Floquet马约拉纳费米子的输运性质，提出实验观测马约拉纳费米子的方法。

拓扑绝缘体是一种具备独特能带结构的全新物质形态，电子在拓扑绝缘体中沿材料的边缘输运，边缘态受到拓扑保护，不易被缺陷和噪声干扰。因此，拓扑量子材料具有极低的能耗和电阻，深入探究拓扑量子输运特性是近年来凝聚态物理领域的研究热点。本项目从紧束缚模型出发推导了圆偏振光作用下石墨烯类二维材料的相关公式，建立了石墨烯类二维材料的Floquet拓扑绝缘体理论模型。我们研究了双层石墨烯中的拓扑输运性质，设计了一个Z字型结构以探索其边缘拓扑输运特性，得到了一个高效的拓扑量子晶体管。此拓扑量子晶体管的输运性质稳定，输运过程无散射和热耗散，不受外界缺陷影响，其导通与关闭状态可以通过外加的垂直电场来控制。我们研究了基于石墨烯的拓扑绝缘体中的自旋转移力矩，器件由三部分构成：普通石墨烯/石墨烯拓扑态/含有铁磁作用的石墨烯,电流经过石墨烯拓扑态部分后被极化从而对铁磁产生自旋转移力矩。此结构可以产生实验可测的自旋转移力矩，且自旋转力矩十分稳定，不会受到拓扑绝缘体形状的影响，无论是边缘缺陷还是材料中的缺陷都不会改变自旋转移力矩的特性。利用圆偏振光对石墨烯能带结构的调制作用，我们设计了可通过光照来控制的场效应晶体管。在石墨烯材料中加入一个顶电极和一个底电极，石墨烯中的载流子浓度和分布可以通过电压来控制，当加入圆偏振光辐照时，我们可以得到一个同时通过外加电压和光照来控制的场效应晶体管。最后，我们研究了石墨烯纳米带与磷烯纳米带中的自旋与谷自旋输运。利用石墨烯纳米结构的非对称性，我们在石墨烯纳米带T型结构中得到了纯谷自旋流；利用铁磁绝缘体的近邻效应，我们在磷烯纳米带中得到了很强的自旋极化和磁阻，并且通过改变外加电压我们可以得到在-100%~100%间转换的自旋极化和磁阻。