Longitudinal acoustic spin and global spin–orbit interaction in vortex beams

Spin and orbital angular momenta (AM) are of fundamental importance in physics. Acoustic waves, as typical longitudinal waves, are often perceived as spin-0 waves. Although spin AM density has been found in acoustics, the total longitudinal spin AM is, however, often vanishing. Here, from a self-consistent theoretical frame, we establish the spin, orbital, and total AM of acoustic vortex beams, and discover that a non-zero integral longitudinal spin AM is carried by the propagating acoustic field. With the longitudinal acoustic spin, we unveil a new mechanism of spin-orbit interaction emerging when a vortex beam is compressed or expanded. Moreover, we reveal the connection and distinction between the acoustic canonical-Minkowski and kinetic-Abraham AM, and prove that only the former is conserved under the corresponding symmetry. Our discovery elucidates new fundamental aspects of spin and orbital AM as well as their interplay in acoustics, which can be extended to other classical waves and may open up new ways for AM-based applications in these systems.