Symmetry-protected topological phases in lattice gauge theories: Topological QED_2

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The interplay of symmetry, topology, and many-body effects in the classification of phases of matter poses a formidable challenge in condensed-matter physics. Such many-body effects are typically induced by inter-particle interactions involving an action at a distance, such as the Coulomb interaction between electrons in a symmetry-protected topological (SPT) phase. In this work, we show that similar phenomena also occur in certain relativistic theories with interactions mediated by gauge bosons, and constrained by gauge symmetry. In particular, we introduce a variant of the Schwinger model or quantum electrodynamics (QED) in 1 + 1 dimensions on an interval, which displays dynamical edge states localized on the boundary. We show that the system hosts SPT phases with a dynamical contribution to the vacuum theta-angle from edge states, leading to a new type of topological QED in 1 + 1 dimensions. The resulting system displays an SPT phase which can be viewed as a correlated version of the Su-Schrieffer-Heeger topological insulator for polyacetylene due to nonzero gauge couplings. We use bosonization and density-matrix renormalization group techniques to reveal the detailed phase diagram, which can further be explored in experiments of ultra-cold atoms in optical lattices.
© 2019 American Physical Society E. E. and G. M. are partially supported by INFN through the project Quantum information technologies in Madrid (QUANTUM). S. P. K. acknowledges the support of STFC Grant No. ST/L000369/1. A. B. acknowledges support from RYC-2016-20066, FIS2015-70856-P, and CAM regional research consortium QUITEMAD+. A. B. thanks T. Byrnes for kindly sharing the document of his PhD thesis [50], A. Celi, P. Silvi, and P. Zoller for interesting discussions, and A. Celi for bringing the work [25] to our attention.
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