[Paper Review] Observation of dislocation-induced topological modes in a three-dimensional acoustic topological insulator

Xue, Haoran, Jia, Ding+7 more|arXiv (Cornell University)|Apr 27, 2021

Topological Materials and Phenomena63 citations

TL;DR

This study experimentally demonstrates dislocation-induced topological helical modes in a three-dimensional acoustic topological insulator, using a stacked 2D acoustic TI with a engineered screw dislocation. Direct spin-resolved field mapping and numerical simulations confirm robust one-dimensional helical modes bound to the dislocation, validating the bulk-dislocation correspondence in a classical 3D topological system.

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.

Motivation & Objective

• To experimentally observe dislocation-induced topological helical modes in a 3D acoustic topological insulator.
• To verify the bulk-dislocation correspondence principle in a classical 3D system with real-space lattice defects.
• To demonstrate robust 1D waveguiding via dislocation-bound modes in a weak 3D topological insulator.
• To validate theoretical predictions of gapless helical modes at screw dislocations using acoustic lattice platforms.

Proposed method

• Constructed a weak 3D acoustic topological insulator by stacking 2D acoustic TIs with vanishing inter-layer coupling.
• Engineered a screw dislocation via a 'cut-and-glue' procedure, modifying couplings to create a spiral staircase pattern in the lattice.
• Designed an acoustic lattice using resonators and coupling tubes with sign-controlled couplings to realize the tight-binding model.
• Performed spin-resolved field mapping to measure spatial distribution and spin texture of the modes.
• Conducted tight-binding and finite-element simulations to corroborate experimental observations.
• Used periodic boundary conditions in simulations to isolate dislocation modes and eliminate surface state interference.

Experimental results

Research questions

• RQ1 Can dislocation-induced helical modes be experimentally observed in a 3D acoustic topological insulator?
• RQ2 Do the observed modes exhibit the predicted spin texture and topological protection?
• RQ3 Is the bulk-dislocation correspondence principle valid in a classical 3D system with a screw dislocation?
• RQ4 Can such dislocation-bound modes serve as robust 1D waveguides in 3D media?
• RQ5 How do the mode profiles and dispersion relate to the topological indices and Burgers vector?

Key findings

• Direct experimental observation of dislocation-induced helical modes in a 3D acoustic TI was achieved via spin-resolved field mapping.
• The spatial distribution of the modes matched theoretical predictions, showing helical edge-like behavior along the dislocation line.
• Numerical simulations confirmed the presence of two-fold degenerate, gapless helical modes within the bulk bandgap.
• The modes were found to be robust and localized along the screw dislocation, with no leakage into the bulk.
• The experimental and simulation results validated the bulk-dislocation correspondence, with B·Gv/2π ≠ 0 confirming topological protection.
• The system demonstrated a complete bandgap of ~100 Hz, within which the helical modes were fully confined and dispersionless over the full gap.