Non-Hermitian topology in static mechanical metamaterials

Programmable nonreciprocal Poynting effect enabled by lattice metamaterials

Shear nonreciprocity, implying unequal shear forces in opposite shear directions, can be achieved by arranging structures asymmetrically. However, the nonreciprocal Poynting effect, i.e., unequal normal stresses induced by the same shear displacements to the left and right, has not been fully explored. We discover the nonreciprocal Poynting effect using a generalized directional truss model. Inspired by this discovery, the cylindrical lattice metamaterials constructed from antisymmetric curled microstructures are used as a case study to generate the nonreciprocal Poynting effect. We develop a design framework that integrates digital generation, finite deformation theory, finite element modeling, and three-dimensional printing to program the nonreciprocal Poynting effect. Applications such as bionic Poynting effect matching, wave energy converter, and unidirectional motion limitation are demonstrated. This framework allows the one-to-one mapping between the torque and normal forces, paving the way for designing soft devices with precise force transmission capabilities.

Effective enhancement of the non-Hermitian corner skin effect in reciprocal photonic crystals

With the rich physical phenomena arising from non-Hermitian systems, the non-Hermitian skin effect (NHSE) has become a current research hotspot. Nowadays, the corner skin effect based on non-reciprocal photonic crystals has been proposed. Considering the complexity of realizing non-reciprocity, the corner skin effect based on reciprocal photonic crystals is well worth investigating. In this Letter, a non-Hermitian reciprocal geometry-dependent corner skin effect based on two-dimensional photonic crystals is presented, which is manifested as the distribution of eigenstates on the corners of a particular geometry by applying open boundary conditions in both directions of photonic crystals. For the better application of the NHSE in the future, such as highly sensitive sensors and lasers, a new, to the best of our knowledge, method that can effectively enhance the performance of the NHSE in photonic crystals is proposed. The method introduces both gain and loss in an ideal photonic crystal to enhance the non-Hermitian specificity of the system, which improves the performance of the non-Hermitian corner skin effect of photonic crystals by 64.5%. Furthermore, this geometry-dependent corner skin effect is corroborated with the spectral topology.