Quantum Spin Hall Effect Topological Phonon Transport in GHz AlN Phononic Crystals

We experimentally demonstrate the quantum spin Hall effect in aluminum nitride (AlN) phononic crystals operating at gigahertz frequencies. The system consists of a patterned AlN membrane supporting topologically nontrivial phononic bands with spin-momentum-locked edge modes. Interdigital transducers (IDTs) are used to generate spin-polarized acoustic excitations, and the resulting wave fields are imaged by transmission-mode microwave impedance microscopy (TMIM). The measurements reveal robust edge transport along domain boundaries and characteristic spin-dependent wave propagation immune to structural disorder. Furthermore, we realize topological functionalities such as spin-selective waveguides and a topological splitter demonstrating controllable phonon routing. Our results establish a chip-scale platform for topological phononics at microwave frequencies and open new possibilities for acoustic signal processing and quantum information transduction.