He Cheng, Sun Xiao-Chen, Liu Xiao-Ping, Lu Ming-Hui, Chen Yulin, Feng Liang, Chen Yan-Feng
National Laboratory of Solid State Microstructures & Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China;
National Laboratory of Solid State Microstructures & Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China; Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China;
Proc Natl Acad Sci U S A. 2016 May 3;113(18):4924-8. doi: 10.1073/pnas.1525502113. Epub 2016 Apr 18.
A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron's spin-1/2 (fermionic) time-reversal symmetry [Formula: see text] However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon's spin-1 (bosonic) time-reversal symmetry [Formula: see text] In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp ([Formula: see text]), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.
拓扑绝缘体是一种内部绝缘但具有时间反演对称性保护的导电边缘态的材料。自近十年前被预测和发现以来，这种对称性保护的拓扑相已在光子学领域中超越电子系统得到探索。电子是自旋为1/2的粒子，而光子是自旋为1的粒子。这两种粒子明显的自旋差异意味着它们相应的对称性根本不同。众所周知，电子拓扑绝缘体受电子的自旋1/2（费米子）时间反演对称性保护[公式：见原文]。然而，由于光子的自旋1（玻色子）时间反演对称性[公式：见原文]，在正常情况下光子拓扑绝缘体不存在同样的保护。在这项工作中，我们报道了一种光子拓扑绝缘体的设计，该设计使用特勒根磁电耦合作为左旋和右旋圆偏振螺旋自旋态的光子赝自旋轨道相互作用。特勒根磁电耦合打破了玻色子时间反演对称性，但由于电磁对偶性，反而产生了一种守恒的类费米子赝时间反演对称性Tp（[公式：见原文]）。令人惊讶的是，我们发现，在这个系统中，螺旋边缘态实际上受这种类费米子赝时间反演对称性Tp保护，而不是受玻色子时间反演对称性Tb保护。这一显著发现有望为理解其他基本粒子拓扑相的对称性保护机制以及寻找拓扑绝缘体的新实现方式开辟一条新路径。
