Fragile Topology and Wannier Obstructions

Hoi Chun Po, Haruki Watanabe, Ashvin Vishwanath

Published 2017-09-19Version 1

Topological phases, such as Chern insulators, are defined in terms of additive indices that are stable against the addition of trivial degrees of freedom. Also, such topology presents an obstruction to representing bands in terms of symmetric, exponentially localized Wannier functions. Here, we address the converse question: Do obstructions to Wannier representations imply stable band topology? We answer this in the negative, pointing out that some bands can also display a distinct type of "fragile topology." Bands with fragile topology admit a Wannier representation if and only if additional trivial degrees of freedom are supplied. We apply this notion to solve a puzzle in diagnosing band topology: A recent work [Nature 547, 298-305 (2017)] made the intriguing suggestion that whenever a so-called "elementary band representation" splits, the two sets of bands, separated by a band gap, are individually topological. Here, we construct a counterexample, defined on the honeycomb lattice, which features a split elementary band. We show that one of the two disconnected bands is completely trivial with exponentially localized, symmetric Wannier functions. This presents a conundrum regarding the nature of the second band, which is resolved by recognizing that it is topological, but in the fragile sense. Our model thus provides a physical example of fragile topology.