We experimentally demonstrate boundary-induced helical bulk states (BI-HBSs) for RF acoustic transport in LiNbO3 thin-film phononic crystals (approximately 175-200 MHz). A boundary-symmetry selection rule at an accidental {\Gamma}-point fourfold degeneracy creates interior bulk channels that couple to wide-aperture interdigital transducers without edge-aperture mismatch. Near-field vibrometry and two-port RF S-parameters confirm low-loss propagation with strongly suppressed backscattering through wavelength-scale defects. The helical band also provides slow-wave, low-dispersion delay and phase control on chip.
@article{arxiv.2502.18385,
title = {Boundary-induced helical bulk acoustic transport in LiNbO3 thin films},
author = {Zhe Li and Zhen-Hui Qin and Shu-Mao Wu and Chen-Bei Hao and Fan-Yun Pan and Hao Yan and Yi-Han He and Yan-Shen Zhou and Xue-Jun Yan and Si-Yuan Yu and Cheng He and Ming-Hui Lu and Yan-Feng Chen},
journal= {arXiv preprint arXiv:2502.18385},
year = {2026}
}