  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.
