低维系统中非局域电阻研究

Since the nonlocal measurement is helpful to discover nontrivial physics that is not easy to detect directly, the nonlocal measurement has now become one of the focus in condensed matter physics. Recent experiments find the signal of giant nonlocal resistance in H-shaped graphene samples due to the spin/valley Hall effect, which exhibits properties contradictive to the classical theory. In this project, based on our previous research experiences in graphene, topological insulator and quantum transport, we propose to investigate the nonlocal resistance in low-dimensional system with the nonequilibrium Green’s function method in the following three aspects: (1) the nonlocal resistance due to the spin Hall effect; (2) the nonlocal resistance due to the valley Hall effect; (3) the nonlocal resistance due to the transport mechanisms protected by topology. We attempt to analyze the origination of the novel properties exhibited by the nonlocal resistance, provide a comprehensive picture of the transport mechanisms that induces the nonlocal resistance, and finally be helpful to experimental confirmation with newly designed devices.

非局域实验方法由于其能够间接探测某些难以直接捕获的非平庸物理机制，近年来逐渐成为凝聚态物理的研究热点之一。最近的实验在H型石墨烯样品中观测到了由自旋/谷霍尔效应诱导的巨大的非局域电阻信号，该信号表现出多种与经典理论相矛盾的性质。申请人基于在石墨烯、拓扑绝缘体、量子输运等相关领域的理论研究积累，计划利用非平衡格林函数方法在以下三个方面深入地研究低维系统中的非局域电阻：（1）由自旋霍尔效应诱导的非局域电阻；（2）由谷霍尔效应诱导的非局域电阻；（3）由受拓扑保护的输运机制诱导的非局域电阻。我们希望能够在理论上分析出非局域电阻奇异性质产生的根源，加深对诱导其产生的量子输运机制的理解，并同时设计装置指导实验验证。

由于非局域探测方法可以得到一些通过常规方法难以观测到的性质，越来越多的研究开始集中于非局域电阻，特别是在低维系统例如石墨烯等材料中。最近的研究提出基于黏性流体力学可以计算出一个涡流的产生，进而解释实验中观测到的非局域电阻在偏离狄拉克点时迅速衰减甚至为负数的现象。甚至有人提出，涡流的存在可以作为系统处于黏性流体的证据。我们的研究证明，经典的弹道输运同样可以支持一个涡流的产生，这个涡流的强弱可以决定非局域电阻的衰减情况。更重要的是，通过与四角晶格计算结果的对比，我们提出实验中观测到的非局域电阻异常现象大多存在于石墨烯等六角晶格材料中的现象，可能源于六角晶格特有的线性色散关系，不同的色散关系会导致电子平均自由程的差异。我们的另一项研究指出，对于Magnus霍尔效应的非局域电阻系统，观测到的Magnus非局域电阻通常会被平庸的信号所掩盖。结合第一性原理的合作，我们通过非平衡格林函数方法证明了一种可以通过旋转系统得到纯净的Magnus非局域电阻的方法。这些研究既是对已有分析的补充，更扩展了人们对于非局域电阻的认识。