[Paper Review] Experimental Realization of Acoustic Chern Insulator

Yifan Zhu, Peng Yugui+5 more|arXiv (Cornell University)|Jan 24, 2018

Topological Materials and Phenomena3 citations

TL;DR

This paper experimentally demonstrates the first acoustic Chern insulator (ACI) by breaking time-reversal symmetry via an angular-momentum-biased resonator array with Lorentz reciprocity violation. Using high Q-factor resonators to enable stable, uniform metafluid flow bias, the system achieves robust, one-way nonreciprocal sound transport at the boundary, topologically protected against defects and scattering, marking a key advance in classical topological acoustics.

ABSTRACT

Topological insulators are new states of matter in which the topological phase originates from symmetry breaking. Recently, time-reversal invariant topological insulators were demonstrated for classical wave systems, such as acoustic systems, but limited by inter-pseudo-spin or inter-valley backscattering. This challenge can be effectively overcome via breaking the time-reversal symmetry. Here, we report the first experimental realization of acoustic topological insulators with nonzero Chern numbers, viz., acoustic Chern insulator (ACI), by introducing an angular-momentum-biased resonator array with broken Lorentz reciprocity. High Q-factor resonance is leveraged to reduce the required speed of rotation. Experimental results show that the ACI featured with a stable and uniform metafluid flow bias supports one-way nonreciprocal transport of sound at the boundaries, which is topologically immune to the defect-induced scatterings. Our work opens up opportunities for exploring unique observable topological phases and developing practical nonreciprocal devices in acoustics.

Motivation & Objective

• To realize a topological insulator in an acoustic system with a nonzero Chern number, enabling robust one-way sound transport.
• To overcome limitations of inter-pseudo-spin and inter-valley backscattering in time-reversal invariant acoustic topological systems.
• To break time-reversal symmetry in a classical acoustic system using angular-momentum-biased resonators to induce nonreciprocal transport.
• To demonstrate topological protection of edge states against defects and scattering in a practical, experimentally feasible setup.
• To open pathways for nonreciprocal devices and novel topological phases in acoustics.

Proposed method

• Implementation of a 2D array of resonators with engineered angular momentum bias to break Lorentz reciprocity.
• Use of high Q-factor resonators to reduce the required rotational speed for achieving effective time-reversal symmetry breaking.
• Design of a metafluid flow bias to induce a synthetic gauge field in the acoustic system.
• Engineering of resonator geometry and coupling to achieve a nontrivial Chern number in the band structure.
• Experimental measurement of sound transmission and edge state propagation under varying defect conditions.
• Characterization of nonreciprocal transmission using directional sound injection and detection at the boundary.

Experimental results

Research questions

• RQ1 Can a topological insulator with a nonzero Chern number be realized in a classical acoustic system?
• RQ2 How can time-reversal symmetry be broken in an acoustic system to enable nonreciprocal edge transport?
• RQ3 Can robust, defect-immune one-way sound propagation be experimentally demonstrated in a phononic structure?
• RQ4 What role does high Q-factor resonance play in enabling effective time-reversal symmetry breaking at feasible rotation speeds?
• RQ5 To what extent are edge states protected against scattering from structural defects in the acoustic Chern insulator?

Key findings

• The experimental system achieved a stable, uniform metafluid flow bias that breaks time-reversal symmetry and induces a nonzero Chern number in the acoustic band structure.
• One-way nonreciprocal sound transport was observed at the boundary, with transmission occurring only in the forward direction under directional excitation.
• Edge states remained robust against various defect-induced scatterings, confirming topological protection.
• High Q-factor resonators enabled effective time-reversal symmetry breaking at experimentally feasible rotational speeds, reducing mechanical requirements.
• The system demonstrated a clear distinction between bulk and edge states, with edge modes propagating unidirectionally along the boundary.
• The experimental results matched theoretical predictions of a Chern insulator phase in acoustics, confirmed by measured transmission spectra and spatial field profiles.