Probing fragile topology with a screw dislocation

TL;DR

This paper demonstrates that fragile topological phases can be experimentally probed through screw dislocations, revealing 1D gapless bound modes and establishing a new bulk-defect correspondence in fragile topological materials.

Contribution

It introduces a method to detect fragile topology via screw dislocations in phononic crystals, despite the absence of gapless edge states, expanding experimental tools for fragile topological phases.

Findings

1D gapless bound modes emerge at screw dislocations due to fragile topology

Experimental observation of topological bound states using pump-probe measurements

Establishment of a bulk-defect correspondence specific to fragile topological phases

Abstract

Fragile topology, akin to twisted bilayer graphene and the exotic phases therein, is a notable topological class with intriguing properties. However, due to its unique nature and the lack of bulk-edge correspondence, the experimental signature of fragile topology has been under debated since its birth. Here, we demonstrate experimentally that fragile topological phases with filling anomaly can be probed via screw dislocations, despite that they do not support gapless edge states. Using a designer hexagonal phononic crystal with a fragile topological band gap, we find that 1D gapless bound modes can emerge at a screw dislocation due to the bulk fragile topology. We then establish a connection between our system and the twisted boundary condition via the gauge invariance principle and illustrate that such an emergent phenomenon is an intrinsic property of fragile topological phases with filling anomaly. We observe experimentally the 1D topological bound states using the pump-probe measurements of their dispersion and wavefunctions, which unveils a novel bulk-defect correspondence of fragile topology and a powerful tool for probing fragile topological phases and materials.