phononspin

Phononspin is a term used in condensed matter physics to describe the angular momentum carried by certain lattice vibrational modes, or phonons, in a crystal. While phonons are quantized normal modes of a lattice and do not possess intrinsic spin in the same sense as elementary particles, some phonon modes are circularly polarized and carry a finite angular momentum. In this sense, the concept of phonon spin (or phonon angular momentum) characterizes the rotational character of specific vibrational excitations.

The angular momentum of phonons arises from the polarization and phase relations of atomic displacements in a mode. In crystals with broken inversion symmetry or strong spin–phonon coupling, vibrational eigenmodes can exhibit circular or elliptical polarization, leading to nonzero angular momentum per phonon. Theoretical descriptions treat phonon angular momentum using phonon creation and annihilation operators together with the mode polarization vectors, allowing interactions with magnetic moments and other angular-momentum carriers.

Experimental interest centers on identifying circularly polarized phonons and their coupling to spins, magnons, or electronic degrees of freedom. Probes include ultrafast optical spectroscopies and Raman or infrared measurements sensitive to mode polarization. Conceptual frameworks have also proposed phenomena such as the phonon spin Hall effect, where phonon angular momentum is transported transverse to a gradient, and the transfer of angular momentum between phonons and magnetic systems.

Phononspin is part of broader efforts in phononics and spintronics to control energy and information flow via vibrational degrees of freedom. Its study depends on material symmetry, temperature, and the strength of spin–phonon interactions, and remains an active area of research.