Realization of Surface-Obstructed Topological Insulators

TL;DR

This paper reports the first experimental realization of three-dimensional surface-obstructed topological insulators using acoustic crystals, demonstrating higher-order topological states with hinge modes without bulk gap closure.

Contribution

It provides the first experimental demonstration of surface-obstructed topological insulators and reveals their unique bulk-boundary physics through acoustic measurements.

Findings

Observation of one-dimensional hinge states within bulk and surface gaps

Evidence that topological phase transition involves surface gap closure

Experimental validation of higher-order topological band topology

Abstract

Recently, higher-order topological insulators have been attracting extensive interest. Unlike the conventional topological insulators that demand bulk gap closings at transition points, the higher-order band topology can be changed without bulk closure and exhibits as an obstruction of higher-dimensional boundary states. Here, we report the first experimental realization of three-dimensional surface-obstructed topological insulators with using acoustic crystals. Our acoustic measurements demonstrate unambiguously the emergence of one-dimensional topological hinge states in the middle of the bulk and surface band gaps, as a direct manifestation of the higher-order band topology. Together with comparative measurements for the trivial and phase-transition-point insulators, our experimental data conclusively evidence the unique bulk-boundary physics for the surface-obstructed band topology. That is, the topological phase transition is determined by the closure of surface gap, rather than by closing the bulk gap. Our study might spur on new activities to deepen the understanding of such elusive topological phases.