Helical Phononic Modes Induced by a Screw Dislocation

Abstract: In this study, we investigate a one-dimensional (1D) unidirectional phononic waveguide embedded within a three-dimensional (3D) hexagonal close-packed phononic crystal, achieved by the introduction of a screw dislocation. This approach does not rely on the non-trivial topological characteristics of the 3D crystal. We discover that this dislocation induces a pair of helical modes, characterized by their orthogonal $x$- and $y$-directional displacements being out of phase by 90 degrees, which results in a distinctive rotational motion. These helical modes demonstrate directional propagation, tightly linked to the helicity of the screw dislocation. Through considerations of symmetry, we reveal that the emergence of these helical modes is governed by the symmetry of the screw dislocation itself. Our findings not only provide insights into the interplay between dislocation-induced symmetry and wave propagation in phononic systems but also open new avenues for designing directionally selective waveguides without relying on the crystal's topological properties.