Observation of dispersion anomalies by design

Abstract: Band structures encode electronic, optical, and acoustic properties of matter and can serve as an essential tool in material discovery and design. Dispersion anomalies -- sharp, non-standard features in the frequency-wavenumber relation -- have been historically correlated with phonon-electron coupling or long-range interaction. Through a combination of experimental, numerical, and analytical methods, we show how magnetic couplings can induce negative stiffness and sculpt dispersion relations to support zero-frequency phonon anomalies at arbitrary, non-zero wavenumbers. Our approach enables the realization of complete wavenumber band gaps without time-modulation, electron-phonon coupling, or long-range interactions. We identify the conditions under which non-differentiable zero-frequency phonons exist away from the high-symmetry points. Our framework generalizes across monoatomic and diatomic lattices, locally resonant metamaterials, non-local systems, as well as higher dimensional crystals. In addition, we report the first passive- or active- experimental observation of wavenumber band gaps in higher dimensions. Our work establishes a new paradigm in dispersion engineering and provides means for understanding wave-matter interaction in both the frequency and wavenumber domains.