Observation of Dislocation-Induced Topological Modes in a Three-Dimensional Acoustic Topological Insulator

Abstract

The interplay between real-space topological lattice defects and the reciprocal-space topology of energy bands can give rise to novel phenomena, such as one-dimensional topological modes bound to screw dislocations in three-dimensional topological insulators. We obtain direct experimental observations of dislocation-induced helical modes in an acoustic analog of a weak three-dimensional topological insulator. The spatial distribution of the helical modes is found through spin-resolved field mapping, and verified numerically by tight-binding and finite-element calculations. These one-dimensional helical channels can serve as robust waveguides in three-dimensional media. Our experiment paves the way to studying novel physical modes and functionalities enabled by topological lattice defects in three-dimensional classical topological materials.

Non-Hermitian dislocation modes: Stability and melting across exceptional points

“…Signatures of NH topology, such as edge states and EPs, have been observed in mechanical systems as well [39][40][41][42], where at least the right eigenstates can be probed directly [41,42]. In mechanical systems both dislocation and disclination lattice defects have been engineered to probe Hermitian topological phases [46][47][48], by introducing hopping phase modulation of π across the line of missing microwave resonators (playing the role of lattice sites). Besides directly probing right eigenvectors, dislocation modes can also be detected from the zeros of the reflection coefficient [40] or mechanical susceptibility [46].…”

“…Recently, NH topological phases have been realized in photonic crystals with gain and/or loss [33][34][35][36][37][38] and in mechanical metamaterials [39][40][41][42]. On these platforms lattice defects have been engineered to probe Hermitian topological phases [43][44][45][46][47][48]. Therefore, our predictions can, in principle, be tested in NH metamaterials.…”

Progress in Topological Mechanics

“…Topological lattice defects [27] have been extensively studied in electronic systems [138][139][140] and classical systems [141][142][143][144][145][146][147][148] over the past decade. The classification of topological lattice defects depends on the holonomy along a closed path around the defect core [25,149].…”

“…The dislocations and disclinations are defined as a gauge flux for translational symmetry and rotational symmetry, respectively [28]. Dislocations and disclinations are mainly investigated on honeycomb lattices and square lattices [149], and it has been demonstrated that they can serve as a bulk probe to detect the topological nature which exhibits topological states [139][140][141][142][143][146][147][148][154][155][156] and/or fractional charges [144,145,157,158] in TCIs protected by crystal symmetries. More recently, topological defect states have been proposed in elastic materials [87,[159][160][161], and then we will discuss the related papers.…”

Observation of degenerate zero-energy topological states at disclinations in an acoustic lattice

“…An important extension of this principle applies to specific topological defects, where the existence of topological states hinges on an interplay between the bulk topology of the lattice and the topological charge of the defect [20][21][22][23][24]. Notable examples include topological states bound to vortices [25][26][27][28], dislocations [29][30][31][32], and disclinations [33][34][35][36][37][38].…”