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arXiv 1403.6491cond-mat.str-el
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反常对称分数次化与曲面拓扑序

Anomalous Symmetry Fractionalization and Surface Topological Order

Xie Chen, Fiona J. Burnell, Ashvin Vishwanath, Lukasz Fidkowski

摘要:

除了具有分数统计,二维拓扑有序态的任意子激发可以以不同的方式实现对称性,导致各种对称性丰富的拓扑(SET)相。虽然对称性分裂必须与任意子的融合和编织规则一致,但并非所有表面上一致的对称性分裂都可以在2D系统中实现。相反,某些“异常”SET只能发生在3D对称保护拓扑(SPT)相的表面上。在本文中,我们描述了一个程序来确定是否一个离散的,现场的,酉对称群$G$的SET是异常或没有。其基本思想是规范的对称性和暴露的异常作为一个一致的拓扑理论相结合的原始任意子和规范通量的障碍。利用Etingof,Nikshych,和Ostrik的结果,我们指出,一类障碍物被捕获的第四上同调组$H^4( G, \,U(1))$,它也精确地标记的三维SPT阶段的集合,与对称群$G$。因此,我们建立了一个一般的体边界之间的异常SET和三维体SPT的表面终止实现它。我们说明了这一想法使用手征自旋液体($U(1)_2$)拓扑秩序与减少对称性$\mathbb{Z}_2 \times \mathbb{Z}_2 \subset SO(3)$,它可以在一个异常的方式作用于半子准粒子。我们构建了精确求解的三维SPT模型,实现了异常的表面终端,并通过计算三圈编织统计证明了它们是非平凡的。还讨论了反酉对称性的可能扩展。

英文摘要:

In addition to possessing fractional statistics, anyon excitations of a 2D topologically ordered state can realize symmetry in distinct ways , leading to a variety of symmetry enriched topological (SET) phases. While the symmetry fractionalization must be consistent with the fusion and braiding rules of the anyons, not all ostensibly consistent symmetry fractionalizations can be realized in 2D systems. Instead, certain `anomalous' SETs can only occur on the surface of a 3D symmetry protected topological (SPT) phase. In this paper we describe a procedure for determining whether an SET of a discrete, onsite, unitary symmetry group $G$ is anomalous or not. The basic idea is to gauge the symmetry and expose the anomaly as an obstruction to a consistent topological theory combining both the original anyons and the gauge fluxes. Utilizing a result of Etingof, Nikshych, and Ostrik, we point out that a class of obstructions are captured by the fourth cohomology group $H^4( G, \,U(1))$, which also precisely labels the set of 3D SPT phases, with symmetry group $G$. We thus establish a general bulk-boundary correspondence between the anomalous SET and the 3d bulk SPT whose surface termination realizes it. We illustrate this idea using the chiral spin liquid ($U(1)_2$) topological order with a reduced symmetry $\mathbb{Z}_2 \times \mathbb{Z}_2 \subset SO(3)$, which can act on the semion quasiparticle in an anomalous way. We construct exactly solved 3d SPT models realizing the anomalous surface terminations, and demonstrate that they are non-trivial by computing three loop braiding statistics. Possible extensions to anti-unitary symmetries are also discussed.

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