  The notion that phonons can carry pseudo-angular momentum has become popular
in the last decade, with recent research efforts highlighting phonon chirality,
Berry curvature of phonon band structure, and the phonon Hall effect. When a
phonon is resonantly coupled to a crystal electric field excitation, a
so-called vibronic bound state forms. Here, we observe angular momentum
transfer of $\delta$Jz = $\pm$1$\hbar$ between phonons and an orbital state in
a vibronic bound state of a candidate quantum spin liquid. This observation has
profound implications for the engineering of phonon band structure topology
through chiral quasiparticle interactions.
