Observation of acoustic spin skyrmion in confined sound field with orbital angular momentum

Topological textures of spin vectors are fundamentally interesting and possess generic features for various vector waves. However, experimental realizations of spin skyrmions are mostly restricted to magnetic materials and optical waves, which remains a significant challenge for airborne sound waves with intrinsic curl-free features. In this work, we propose a concentric cylindrical metastructure to construct spin skyrmion textures with subwavelength scale excited by an acoustic spin source. By acoustic spin-orbital coupling, we achieve multi-order confined Bessel spoof acoustic surface waves with orbital angular momentum on the metastructure surface, experimentally observe the acoustic spin skyrmion configurations with subwavelength scale, and verify the topologically protection for introducing the defects. The achievement of acoustic spin skyrmions not only expands the family of topological textures in sound but also provides an avenue to manipulating small particles by controlling structural acoustic fields.