Robustness of quantized transport through edge states of finite length: Imaging current density in Floquet topological versus quantum spin and anomalous Hall insulators
Robustness of quantized transport through edge states of finite length: Imaging current density in Floquet topological versus quantum spin and anomalous Hall insulators
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DOI：
10.1103/physrevresearch.2.033438
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发表时间：
2020-06
影响因子：
4.2
通讯作者：
U. Bajpai;M. Ku;B. Nikolić
中科院分区：
文献类型：
--
作者：
U. Bajpai;M. Ku;B. Nikolić
拓扑绝缘体（TI）的理论分析传统上集中在具有带隙的无限均质晶体和其波函数的非平凡拓扑，或者其边界具有表面或边缘金属态的无限导线上。然而，实验设备包含连接到正常金属（NM）引线的有限尺寸拓扑区域，这就提出了一个问题，即纵向电导的量化精度如何以及电子如何从拓扑平凡的NM引线跃迁到边缘态。这对于最近获得的二维（2D）Floquet TI来说是特别紧迫的问题，其中电子从与时间无关的NM流到与时间相关的边缘状态-Floquet TI最近的实验实现使用由圆偏振光照射的石墨烯，没有表现出量子化的纵向或霍尔电导。在这里，我们采用电荷守恒的解决方案的Floquet不等绿色功能（NEGFs）的辐照石墨烯纳米计算纵向两端电导，以及空间分布的局部电流密度的电子传播从NM导致Floquet TI。在Floquet TI的情况下，体电流密度和边缘局部电流密度对总电流的贡献相等，这导致纵向电导低于预期的量化平台，边缘空位略微降低。我们提出了两个实验方案，以检测共存的体积和边缘电流密度内Floquet TI：（一）钻纳米孔在内部的照射区域的石墨烯将诱导背散射的体积电流密度，从而减少纵向电导由$\sim 28$%;（二）成像的磁场产生的局部电流密度使用金刚石NV中心。
The theoretical analysis of topological insulators (TIs) has been traditionally focused on infinite homogeneous crystals with band gap in the bulk and nontrivial topology of their wavefunctions, or infinite wires whose boundaries host surface or edge metallic states. However, experimental devices contain finite-size topological region attached to normal metal (NM) leads, which poses a question about how precise is quantization of longitudinal conductance and how electrons transition from topologically trivial NM leads into the edge states. This is particularly pressing issues for recently conjectured two-dimensional (2D) Floquet TI where electrons flow from time-independent NM leads into time-dependent edge states---the very recent experimental realization of Floquet TI using graphene irradiated by circularly polarized light did not exhibit either quantized longitudinal or Hall conductance. Here we employ charge conserving solution for Floquet-nonequlibrium Green functions (NEGFs) of irradiated graphene nanoribbon to compute longitudinal two-terminal conductance, as well as spatial profiles of local current density as electrons propagate from NM leads into the Floquet TI. In the case of Floquet TI both bulk and edge local current densities contribute equally to total current, which leads to longitudinal conductance below the expected quantized plateau that is slightly reduced by edge vacancies. We propose two experimental schemes to detect coexistence of bulk and edge current densities within Floquet TI: (i) drilling a nanopore in the interior of irradiated region of graphene will induce backscattering of bulk current density, thereby reducing longitudinal conductance by $\sim 28$%; (ii) imaging of magnetic field produced by local current density using diamond NV centers.