Direct Observation of All-Flat Bands Phononic Metamaterials

Flat lines within a band structure represent constant frequency bands for all momentum values (i.e., they maintain zero group velocity for all wave numbers). These bands lead to energy localization near the source with very high intensity compared to regular pass bands. An all-flat photonic band structure was observed very recently. However, to date, due to the limitations on coupling stiffness ratios and the vectorial nature of phonon propagation, no existing metamaterial exhibits an all-flat band (AFB) for phonons. In this Letter, we utilize disks with magnetic couplings as a platform to realize an all-flat phononic band structure. We elucidate the required design methodology to obtain an AFB phononic band structure and report on the first direct observation of such metamaterials. Our design methodology can enable the realization of advanced materials with exotic wave control capabilities.