Non-Hermitian chiral surface waves in disordered odd solids

TL;DR

This paper demonstrates that disordered odd elastic solids can host non-Hermitian chiral surface waves with enhanced localization and stability, revealing new ways to control wave propagation in active, disordered materials.

Contribution

It introduces a minimal model showing that torque-driven disordered odd solids support unique, stable chiral surface waves, expanding understanding beyond ordered systems.

Findings

Disordered odd solids support non-Hermitian chiral surface waves.

Surface waves exhibit stronger localization and stable boundary velocity.

Active torques enable control over non-Hermitian surface wave properties.

Abstract

Chiral surface waves are surface-localized modes that propagate unidirectionally along a boundary, enabling directed transport and minimal back-scattering. While first identified in quantum systems, they were recently shown to emerge in classical metamaterials in the presence of `odd elasticity'. Owing to the non-reciprocality of odd elasticity, these waves exhibit growing amplitudes during propagation, reminiscent of the non-Hermitian skin effect. To date, studies of odd elastic systems have mainly focused on ordered structures. Whether structurally-disordered materials can host non-Hermitian chiral surface waves (NHCSW) remains unexplored. We address this question using a minimal model of torque-driven disordered odd solids. Such solids are abundant, from biological gels such as the cytoskeleton driven by motor-proteins to synthesized systems such as magnetic colloidal gels. We find that torque-driven disordered odd solids have unique NHCSW with stronger surface localization and stable boundary velocity, in contrast to previous lattice models of odd solids. These distinct features stem from an intrinsic interplay between boundary torques and odd elasticity in torque-driven odd solids. Our results offer a new strategy to control NHCSW using active torques.