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Coupled-wire description of surface ADE topological order

Bo Han and Jeffrey C. Y. Teo
Phys. Rev. B 99, 235102 – Published 3 June 2019

Abstract

Symmetry-protected and symmetry-enriched topological (SPT/SET) phases in three dimensions are quantum systems that support nontrivial two-dimensional (2D) surface states. These surface states develop finite excitation energy gaps when the relevant symmetries are broken. On the other hand, one-dimensional (1D) gapless modes can populate along interfaces that separate adjacent gapped surface domains with distinct symmetry-breaking orders. A surface strip pattern in general reduces the low-energy SPT/SET surface degrees of freedom onto a 2D array of gapless 1D channels. These channels can be coupled to one another by quasiparticle tunneling, and these interwire interactions collectively provide an effective description of the surface state. In this paper, we study a general class of symmetry-preserving or -breaking SPT/SET surface states that admit finite excitation energy gaps and Abelian topological orders via the coupled-wire construction. In particular, we focus on the prototype Abelian surface topological orders that fall under the ADE classification of simply laced Lie algebras. We also elaborate on the emergent symmetry and duality properties of the coupled-wire models.

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  • Received 30 July 2018
  • Revised 12 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.235102

©2019 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Bo Han1 and Jeffrey C. Y. Teo2

  • 1Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
  • 2Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA

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Issue

Vol. 99, Iss. 23 — 15 June 2019

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Images

  • Figure 1
    Figure 1

    Coupled-wire description of a topological surface state. (a) Emergence of surface channels through alternating symmetry breaking. (b) Gapless surface state resulting from uniform competing interchannel backscattering Hbc. (c) Surface gapping through channel bipartition and noncompeting interchannel dimerization Hdimer.

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  • Figure 2
    Figure 2

    A string of “quasilocal” operators (19) creates a pair of fractional surface excitations in the form of a kink and antikink pair of the sine-Gordon order parameter 2Θy1/2(x).

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  • Figure 3
    Figure 3

    A hc/2e flux vortex in the topological superconducting bulk associates to a string of vertex operators on the surface (represented by the blue stars) and creates a pair of π-kink excitations (red dots).

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  • Figure 4
    Figure 4

    Duality transformation of the sine-Gordon term on the u plane. D̂ is the duality operator. Under D̂, points on the circle with radius |u| are reflected with respect to the real axis. P,Q are self-dual points. P describes a gapless point, which can be seen in Fig. 5. Other points on the circle describe gapped phases.

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  • Figure 5
    Figure 5

    The ground-state expectation values of 2Θy+1/2 that minimize the sine-Gordon Hamiltonian (43) for (a) ϑ=0, (b) ϑ=3π/5, and (c) ϑ=π. The plots are taken over the fundamental region in R3 modulo the Haldane dual lattice LΘSU(4) in (41). The sine-Gordon Hamiltonian generically has a finite-energy gap and a single minimum for π<ϑ<π. At ϑ=π, there are gapless Goldstone modes on the boundary of the fundamental region.

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  • Figure 6
    Figure 6

    Pictorial illustration of the duality transformation in Eq. (50) or (51). Two flux quanta from + and attached to each pair of wires.

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