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Title: Disconnected elementary band representations, fragile topology, and Wilson loops as topological indices: An example on the triangular lattice

Journal Article · · Physical Review. B
 [1];  [2];  [3];  [4]
  1. Univ. of Illinois at Urbana-Champaign, IL (United States); Donostia International Physics Center (Spain); DOE/OSTI
  2. Chinese Academy of Sciences (CAS), Beijing (China); Princeton Univ., NJ (United States)
  3. Stony Brook Univ., NY (United States); Flatiron Institute, New York, NY (United States)
  4. Princeton Univ., NJ (United States); Freie Univ. Berlin (Germany); Max Planck Inst. of Microstructure Physics, Halle (Germany)

In this study, we examine the topological phases that can arise in triangular lattices with disconnected elementary band representations. We show that, although these phases may be “fragile” with respect to the addition of extra bands, their topological properties are manifest in certain nontrivial holonomies (Wilson loops) in the space of nontrivial bands. We introduce an eigenvalue index for fragile topology, and we show how a nontrivial value of this index manifests as the winding of a hexagonal Wilson loop; this remains true even in the absence of time-reversal or sixfold rotational symmetry. Additionally, when time-reversal and twofold rotational symmetry are present, we show directly that there is a protected nontrivial winding in more conventional Wilson loops. Crucially, we emphasize that these Wilson loops cannot change without closing a gap to the nontrivial bands. By studying the entanglement spectrum for the fragile bands, we comment on the relationship between fragile topology and the “obstructed atomic limit” of Bradlyn et al. [Nature (London) 547, 298 (2017)]. We conclude with some perspectives on topological matter beyond the K-theory classification.

Research Organization:
Princeton Univ., NJ (United States)
Sponsoring Organization:
USDOE Office of Science (SC); National Science Foundation
Grant/Contract Number:
SC0016239
OSTI ID:
1612431
Journal Information:
Physical Review. B, Journal Name: Physical Review. B Journal Issue: 4 Vol. 99; ISSN 2469-9950
Publisher:
American Physical Society (APS)Copyright Statement
Country of Publication:
United States
Language:
English

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Twisted bulk-boundary correspondence of fragile topology journal February 2020
Strong and fragile topological Dirac semimetals with higher-order Fermi arcs text January 2020
Fragile topological phases in interacting systems text January 2018
Band topology in classical waves: Wilson-loop approach to topological numbers and fragile topology text January 2019
Diagnosis of topological nodal lines with nontrivial monopole charge in the presence of rotation symmetries text January 2019
Stiefel-Whitney classes and topological phases in band theory text January 2019
Fractional disclination charge in two-dimensional $C_n-$symmetric topological crystalline insulators text January 2019
Classification of crystalline insulators without symmetry indicators: atomic and fragile topological phases in twofold rotation symmetric systems text January 2019
Fractional corner charges in spin-orbit coupled crystals text January 2019
Strong and Fragile Topological Dirac Semimetals with Higher-Order Fermi Arcs text January 2019
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