We investigate theoretically and experimentally a nonlocal micropolar metabeam shunted with piezoelectric elements modulated by electronic feed-forward control. Since the nonlocal feed-forward control breaks inversion symmetry, the proposed metabeam supports nonreciprocal flexural wave propagation, featuring unidirectional amplification/attenuation and non-Hermitian skin effect. The unidirectional wave propagation is attributed to the energy conversion between electronic and mechanical parts. Also, the nonlocal metabeam is capable of realizing mechanical roton-like dispersion whose reciprocity can be broken by the programmable feed-forward control. The nonlocal micropolar metabeam could pave the ways for designing non-Hermitian topological mechanical insulators and metamaterials.
Inspired by the optical and acoustic non-Hermitian 𝒫𝒯 symmetric system, a 𝒫𝒯 symmetric metamaterial beam for flexural waves is proposed here, based on shunted piezoelectric patches. Positive and negative shunting resistances are the key to constructing the balanced loss and gain components. The associated asymmetric flexural wave scattering, 𝒫𝒯 phase transition and exceptional points are then investigated in both analytical and numerical ways. In addition, properties of exceptional points are tunable in our proposed system, simply owing to the variable shunting parameters. Our design may contribute to asymmetric wave control, enhanced sensing, amplification, and localization of flexural waves.
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