Observation of Frozen Mode of Stationary-Inflection-Point Dynamics in Nonlocal Phononic Metamaterials

We report a new type of phononic metamaterials with non-local interactions. Our designs demonstrate a unique and fundamental advantage in achieving any arbitrarily specified dispersion band, which governs the propagation of elastic waves. In particular, we fabricate a sample with a stationary inflection point (SIP a.k.a undulation point) where both the first and second derivatives of the dispersion curve vanish. We perform wave-propagation testing on the sample and observe the localized and non-spreading mode, which is distinctive from regular zero-group-velocity (ZGV) modes. This breaks the restriction on vibro-elastic energy localization and may open a new avenue to design metamaterials to manipulate non-propagating waves.