Strong-to-weak spontaneous breaking of 1-form symmetry and intrinsically mixed topological order
Carolyn Zhang, Yichen Xu, Jian-Hao Zhang, Cenke Xu, Zhen Bi, Zhu-Xi Luo
Abstract
Topological orders in ($2+1$) dimensions are spontaneous symmetry-breaking (SSB) phases of 1-form symmetries in pure states. The notion of symmetry is further enriched in the context of mixed states, where a symmetry can be either ``strong'' or ``weak.'' In this work, we apply a R\'enyi-2 version of the proposed equivalence relation by [S. Sang et al. (unpublished)] on density matrices that is slightly finer than two-way channel connectivity. This equivalence relation distinguishes general 1-form strong-to-weak SSB (SW-SSB) states from phases containing pure states, and therefore labels SW-SSB states as ``intrinsically mixed.'' According to our equivalence relation, two states are equivalent if and only if they are connected to each other by finite Lindbladian evolution that maintains analytically varying, finite R\'enyi-2 Markov length. We then examine a natural setting for finding such density matrices: disordered ensembles. Specifically, we study the toric code with various types of disorders and show that in each case, the ensemble of ground states corresponding to different disorder realizations forms a density matrix with different strong and weak SSB patterns of 1-form symmetries, including SW-SSB. Furthermore, we show by perturbative calculations that these disordered ensembles form stable ``phases'' in the sense that they exist over a finite parameter range, according to our equivalence relation.