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Experimental Observation of Nonreciprocal Waves in a Resonant Metamaterial Beam

Space-time-varying materials pledge to deliver nonreciprocal dispersion in linear systems by inducing an artificial momentum bias. Although such a paradigm eliminates the need for actual motion of the medium, experimental realization of space-time structures with dynamically changing material properties has been elusive. In this letter, we present an elastic metamaterial that exploits stiffness variations in an array of geometrically phase-shifted resonators -- rather than external material stimulation -- to induce a temporal modulation. We experimentally demonstrate that the resulting bias breaks time-reversal symmetry in the resonant metamaterial, and achieves a nonreciprocal tilt of dispersion modes within dynamic modulation regimes.