Observation of Higher-Dimensional Point-Gap Bulk-Boundary Correspondence

Area of Science:

• Condensed Matter Physics
• Topological Physics
• Acoustic Metamaterials

Background:

• The bulk-boundary correspondence (BBC) links a material's bulk topological properties to its boundary behavior.
• Existing BBC frameworks primarily address line-gap topology, leaving point-gap topology less understood.
• Non-Hermitian systems, crucial for realistic physical phenomena, require extensions to topological concepts.

Purpose of the Study:

• To experimentally demonstrate higher-dimensional point-gap bulk-boundary correspondence (BBC).
• To investigate topological phases in non-Hermitian systems using an acoustic platform.
• To provide evidence for topological edge states and the non-Hermitian skin effect in point-gap topological insulators.

Main Methods:

• Implementation of a compact two-dimensional point-gap topological insulator using a versatile acoustic platform.
• Characterization of the topological insulator by its nontrivial open-bulk Chern number.
• Experimental visualization of the point-gap spectrum using a mixed-dimensional acoustic setup (real-space and synthetic-momentum).

Main Results:

• Direct visualization of the characteristic point-gap spectrum.
• Observation of topological edge states in a 2D sample with specific boundary conditions.
• Observation of the bipolar non-Hermitian skin effect in a distinct 2D sample.

Conclusions:

• The experimental results provide decisive evidence for the point-gap bulk-boundary correspondence.
• This work establishes a crucial missing pillar in the field of non-Hermitian topological physics.
• The acoustic platform offers a versatile tool for exploring advanced topological phenomena.