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arXiv 2212.09779cond-mat.str-el
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(3+1)维拓扑序的带隙边界

Gapped boundaries of (3+1)d topological orders

Zhu-Xi Luo

AI总结:

本文研究(3+1)维拓扑序带隙边界的分类问题,提出按弦状激发能否终止于边界来定义边界类,并以玻色环面码为例构造了超出已知三种边界的大量新边界,揭示其与有限群子群的对应关系。

摘要:

给定一个(3+1)维拓扑序(TO)的带隙边界,可以在其上堆叠一个解耦的(2+1)维拓扑序,从而得到另一种边界理论。我们是否应该将这两个边界视为“不同的”?一个自然的选择是“不”。(3+1)维拓扑序的不同带隙边界类别应当对这些解耦的(2+1)维拓扑序取模来定义。但这是否足够?我们研究了将(3+1)维拓扑序的边界耦合到额外的(2+1)维拓扑序或分形子系统的可能性,这会导致更多可能的带隙边界。通常,体中的点状激发在接触边界时会变成所添加的(2+1)维相中的激发,而体中的弦状激发可以终止于边界,但其端点会被(2+1)维相中的其他激发所“修饰”。为了给(3+1)维拓扑序的带隙边界定义一个良好的“类”,我们选择同样对这些不同的修饰取商。我们通过能够终止于边界的弦状激发来刻画一类带隙边界,无论其端点是什么。一个具体的例子是(3+1)维玻色环面码。利用群上同调和范畴论,此前已经发现了三种带隙边界:粗糙边界、光滑边界和扭曲光滑边界。我们可以构造出远超这些的更多带隙边界,它们都自然地分为两类,对应于$m$-弦能否终止于边界。根据这一分类,此前发现的三种边界被归为{粗糙}、{光滑,扭曲光滑}。对于一个由有限群$G$刻画的(3+1)维拓扑序,不同的类对应于$G$的不同子群。我们从多个视角阐释了这一物理图像,包括耦合层构造、Walker-Wang模型和场论。

英文摘要:

Given a gapped boundary of a (3+1)d topological order (TO), one can stack on it a decoupled (2+1)d TO to get another boundary theory. Should one view these two boundaries as "different"? A natural choice would be no. Different classes of gapped boundaries of (3+1)d TO should be defined modulo these decoupled (2+1)d TOs. But is this enough? We examine the possibility of coupling the boundary of a (3+1)d TO to additional (2+1)d TOs or fractonic systems, which leads to even more possibilities for gapped boundaries. Typically, the bulk point-like excitations, when touching the boundary, become excitations in the added (2+1)d phase, while the string-like excitations in the bulk may end on the boundary but with endpoints dressed by some other excitations in the (2+1)d phase. For a good definition of "class" for gapped boundaries of (3+1)d TO, we choose to quotient out the different dressings as well. We characterize a class of gapped boundaries by the string-like excitations that can end on the boundary, whatever their endpoints are. A concrete example is the (3+1)d bosonic toric code. Using group cohomology and category theory, three gapped boundaries have been found previously: rough boundary, smooth boundary and twisted smooth boundary. We can construct many more gapped boundaries beyond these, which all naturally fall into two classes corresponding to whether the $m$-string can or cannot end on the boundary. According to this classification, the previously found three boundaries are grouped as {rough}, {smooth, twisted smooth}. For a (3+1)d TO characterized by a finite group $G$, different classes correspond to different subgroups of $G$. We illustrate the physical picture from various perspectives including coupled layer construction, Walker-Wang model and field theory.

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