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arXiv 1201.2648cond-mat.str-elhep-thmath-phmath.MPquant-ph
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相互作用费米子的对称保护拓扑序--费米子拓扑非线性$σ$模型和一种特殊的群超上同调理论

Symmetry-protected topological orders for interacting fermions -- Fermionic topological nonlinear $σ$ models and a special group supercohomology theory

Zheng-Cheng Gu, Xiao-Gang Wen

摘要:

对称性保护拓扑相(SPT)是具有对称性$G$的带隙短程纠缠量子相,如果打破对称性,它们都可以光滑地连接到平凡的乘积态.它已被证明,一个大类的相互作用玻色子SPT相位可以系统地描述的群上同调理论。本文引入了一个(特殊的)群上同调理论,它是标准群上同调理论的推广。我们证明了一大类短程相互作用的费米子SPT相位可以用群超上同调理论来描述。利用超上循环的数据,我们可以得到相应的费米子SPT相的理想基态波函数。我们还可以得到实现SPT相的体哈密顿量,以及边界有效哈密顿量的反常(即非在位)对称性。边界上的反常对称性意味着对称边界对于1+1D边界必须是无隙的,并且在1+1D之外必须是无隙的或拓扑有序的。作为这一一般结果的应用,我们构造了一个新的SPT相在3D中,相互作用的费米子超导体与共面自旋序(具有$T^2=1$时间反演$Z_2^T$和费米子数奇偶$Z_2^f$对称性描述的一个完整的对称群$Z_2^T\times Z_2^f$)。这样的费米子SPT态既不能由自由费米子实现,也不能由相互作用玻色子(由费米子对形成)实现,因此不包括在自由费米子的K理论分类或相互作用玻色子的群上同调描述中。我们还构造了具有全对称群$Z_2\times Z_2^f$的三个相互作用的二维费米子SPT相。如果对称性没有被破坏,这些二维费米子SPT相都具有中心电荷$c=1$无隙边缘激发。

英文摘要:

Symmetry-protected topological (SPT) phases are gapped short-range-entangled quantum phases with a symmetry $G$, which can all be smoothly connected to the trivial product states if we break the symmetry. It has been shown that a large class of interacting bosonic SPT phases can be systematically described by group cohomology theory. In this paper, we introduce a (special) group supercohomology theory which is a generalization of the standard group cohomology theory. We show that a large class of short-range interacting fermionic SPT phases can be described by the group supercohomology theory. Using the data of super cocycles, we can obtain the ideal ground state wave function for the corresponding fermionic SPT phase. We can also obtain the bulk Hamiltonian that realizes the SPT phase, as well as the anomalous (ie, non-on-site) symmetry for the boundary effective Hamiltonian. The anomalous symmetry on the boundary implies that the symmetric} boundary must be gapless for 1+1D boundary, and must be gapless or topologically ordered beyond 1+1D. As an application of this general result, we construct a new SPT phase in 3D, for interacting fermionic superconductors with coplanar spin order (which have $T^2=1$ time-reversal $Z_2^T$ and fermion-number parity $Z_2^f$ symmetries described by a full symmetry group $Z_2^T\times Z_2^f$). Such a fermionic SPT state can neither be realized by free fermions nor by interacting bosons (formed by fermion-pairs), and thus are not included in the K-theory classification for free fermions or group cohomology description for interacting bosons. We also construct three interacting fermionic SPT phases in 2D with a full symmetry group $Z_2\times Z_2^f$. Those 2D fermionic SPT phases all have central-charge $c=1$ gapless edge excitations, if the symmetry is not broken.

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