Directional propagation of interface modes in topological acoustic metamaterials via spin-momentum locking

Topological acoustics enables backscattering-immune wave transport along domain interfaces, whose directionality can be deterministically controlled through spin-momentum locking of the excitation source. In this work, we computationally demonstrate a monolithic two-dimensional Ge₂Sb₂Te₅ (GST) phononic crystal plate in which hexagonally patterned crystalline inclusions are embedded within an amorphous GST host, where the impedance contrast between the two phases opens a topological bandgap. Here we show that the overlap integral between an external excitation source and the Bloch eigenstates governs directional selectivity. The source position and phase determine which topological pseudospin channel is excited, enabling deterministic routing through spin-momentum locking. Our system exploits [Formula: see text] symmetry, which supports degenerate [Formula: see text]-type and [Formula: see text]-type orbital modes at the [Formula: see text] point serving as pseudospin degrees of freedom. When a single harmonic force is applied, it projects onto both pseudospin channels, yielding bidirectional propagation. By contrast, a quadrature phased force pair on neighboring inclusions generates a rotating displacement field whose coupling to one pseudospin state identically vanishes, locking propagation to a single direction. Swapping the force positions reverses the routing direction, and this reversal is spatially invariant across the interface, providing evidence of spin-momentum locking. By varying only the source configuration, the same interface operates as a bidirectional waveguide, unidirectional isolator, or selective router. These results demonstrate how spin-momentum locking can serve as an efficient mechanism for directional selectivity of topological interface states in monolithic structures, with relevance to on-chip acoustic signal routing and frequency-selective wave filtering.