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arXiv:1501.01313 · Physical review. B./Physical review. B

Classification of symmetry-protected phases for interacting fermions in two dimensions

Meng Cheng, Zhen Bi, Yi‐Zhuang You, Zheng‐Cheng Gu

Published May 9, 2018• 73 citations
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Abstract

The authors deal with the fundamental problem of a rigorous classification of quantum topological phases. Here, they extend to the realm of interacting fermions in two dimensions techniques, developed for bosons, whereby they introduce extrinsic defects carrying symmetry fluxes into the topological phase. The result is a complete classification of symmetry-protected topological phases of interacting fermions in two dimensions. In addition, the classification of interacting fermion systems with time-reversal symmetry is also developed. The advances, reported here, are immediately relevant for the selection of reliable candidates for topologically protected storage and communication.

DOI
10.1103/physrevb.97.205109

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The authors deal with the fundamental problem of a rigorous classification of quantum topological phases. Here, they extend to the realm of interacting fermions in two dimensions techniques, developed for bosons, whereby they introduce extrinsic defects carrying symmetry fluxes into the topological phase. The result is a complete classification of symmetry-protected topological phases of interacting fermions in two dimensions. In addition, the classification of interacting fermion systems with time-reversal symmetry is also developed. The advances, reported here, are immediately relevant for the selection of reliable candidates for topologically protected storage and communication.
Recently, it has been shown that two-dimensional bosonic symmetry-protected topological(SPT) phases with on-site unitary symmetry GGG can be completely classified by the group cohomology class H3(G,U(1))H^3(G, \mathrm{U}(1))H3(G,U(1)). Later, group super-cohomology class was proposed as a partial classification for SPT phases of interacting fermions. In this work, we revisit this problem based on the mathematical framework of GGG-extension of unitary braided tensor category(UBTC) theory. We first reproduce the partial classifications given by group super-cohomology, then we show that with an additional H1(G,Z2)H^1(G, \mathbb{Z}_2)H1(G,Z2​) structure, a complete classification of SPT phases for two-dimensional interacting fermion systems for a total symmetry group G×Z2fG\times\mathbb{Z}_2^fG×Z2f​ can be achieved. We also discuss the classification of interacting fermionic SPT phases protected by time-reversal symmetry.
Symmetry-protected topological (SPT) phases (e.g., the well known topological insulators) are a class of energetically gapped condensed matter systems that exhibit interesting topological properties only in the presence of certain global symmetries. While a great understanding of noninteracting fermionic SPT phases has been achieved recently, interacting SPT phases, in particular those that exist only in strongly interacting systems, are much less understood in general. Here, the authors study strongly interacting fermionic SPT phases in two spatial dimensions with finite Abelian unitary symmetries and provide a potentially complete classification of them.
The construction and classification of symmetry-protected topological (SPT) phases in interacting bosonic and fermionic systems have been intensively studied in the past few years. Very recently, a complete classification and construction of space-group SPT phases were also proposed for interacting bosonic systems. In this Rapid Communication, we attempt to generalize this classification and construction scheme systematically into interacting fermion systems. In particular, we construct and classify point-group SPT phases for two-dimensional (2D) interacting fermion systems via lower-dimensional block-state decorations. We discover several intriguing fermionic SPT states that can only be realized in interacting fermion systems (i.e., not in free-fermion or bosonic SPT systems). Moreover, we also verify the recently conjectured crystalline equivalence principle for 2D interacting fermion systems. Finally, the potential experimental realization of these different classes of point-group SPT phases in 2D correlated superconductors is addressed.
We give a brief overview on interacting symmetry-protected topological (SPT) phases. In particular, we will focus on various cases where the classification of non-interacting fermionic systems breaks down in the presence of interactions. The examples include topological (and trivial) superconductors in one, two and three spatial dimensions, protected by various symmetries such as time-reversal, spin parity, and reflection symmetries. Common to these examples is the ‘collapse’ of the non-interacting classification, where an integer classification (‘’) at non-interacting level reduces to or in the presence of interactions. We make use of these examples to illustrate various approaches to interacting SPT phases.
We give a brief overview on interacting symmetry-protected topological (SPT) phases. In particular, we will focus on various cases where the classification of non-interacting fermionic systems breaks down in the presence of interactions. The examples include topological (and trivial) superconductors in one, two and three spatial dimensions, protected by various symmetries such as time-reversal, spin parity, and reflection symmetries. Common to these examples is the ‘collapse’ of the non-interacting classification, where an integer classification (‘ ’) at non-interacting level reduces to or in the presence of interactions. We make use of these examples to illustrate various approaches to interacting SPT phases.