Bound States at Partial Dislocation Defects in 2D and 3D High-order Topological Insulator Metamaterials

Date:

S. S. Yamada (Presenting Author), T. Li, M. Lin, C. W. Peterson, T. L. Hughes, G. Bahl

Abstract

Topological insulators (TIs) are unique materials with gapped bulk band structure and conductive in-gap states localized to their boundaries. Higher-order topological insulators (HOTIs) are a recently discovered class of topological materials that have robust protected states localized to their higher-order boundaries (e.g., corners rather than edges or surfaces). In this work we experimentally demonstrate that a new type of crystalline defect known as a partial dislocation can robustly trap bound states deep within the bulk of HOTIs both in 2D and 3D. This defect can also function as a bulk probe of topology.

The special boundary states of TIs arise as a consequence of the material’s bulk topology (i.e., non-trivial properties of a material that do not change unless the bulk band gap closes), and should, in principle, appear within the bulk band gap. In crystalline TIs, however, it is possible for these boundary modes to spectrally coincide with bulk modes due to surface loading, contamination, or decoration. This complicates detection since many experimental methods rely on the observation of both spatial and spectral localization of the topological boundary modes. It is therefore of value to explore other bulk probes that are not affected by surface phenomena

In this context, bulk defects of crystalline symmetries––such as disclinations and dislocations have been demonstrated to act as bulk probes of topology, for both TIs and HOTIs, as they trap fractional charges and bound states at their defect cores. Here we explore the similar function of partial dislocation defects in both 2D quadrupole and 3D octupole HOTIs using modular electric circuit metamaterials. Our findings provide a pathway to selectively embed 0D bound states within the bulk of HOTIs, which could prove useful for engineering applications such as topological lasing.