Do phonons carry spin?

Abstract

Recently the question of phonon angular momentum has been raised : phonon angular momentum in a magnetic crystal could play an important role in the angular momentum balance in the Einsteinde Haas effect. This proposition is quite natural and understandable, however the idea of phonon spin seems ambiguous in the literature. In 1988 McLellan presented phonon angular momentum states for circular and elliptical polarizations of phonons that represent orbital angular momentum and also pointed out analogy to isotropic quantum harmonic oscillator. In view of its fundamental importance we investigate this issue and conclude that phonon spin hypothesis is physically untenable, and the phonon angular momentum has to be interpreted as orbital. The photon-phonon analogy is also misleading. It is argued that Cosserat elastic medium having internal torque may have lattice excitations different from phonons having spin one or spin two that we term as Cosseratons. In addition a new source of angular momentum balance originationg from anomalous electron magnetic moment is suggested.

Damping of Magnetization Dynamics by Phonon Pumping

“…The angular momentum of phonons [26,27] has recently come into focus again in the context of the Einstein-de Haas effect [28] and spin-phonon interactions in general [29]. The interpretation of the phonon angular momentum in terms of orbital and spin contributions [29] has been challenged [30], a discussion that bears similarities with the interpretation of the photon angular momentum [31]. In our opinion this distinction is rather semantic since not required to arrive at concrete results.…”

Magnon-phonon conversion experiment and phonon spin

“…We remark that in spite of the usage of phonon polarization in the literature [13], and transverse polarization in the designation of creation and annihilation operators phonon spin and vector nature of the phonon field is nowhere mentioned. To avoid confusion, it has to be understood that scalar field could possess well defined orbital angular momentum and laser light beams with sharp orbital angular momentum have been extensively studied in the literature, see references in [4]. Longitudinal modes have no spin or orbital angular momentum, however linearly polarized light could possess orbital angular momentum but not spin.…”

“…Next using wavevector resolved Brillouin light scattering experiment the measurements show i) magnon-phonon conversion with constant energy and linearly varying momentum, and ii) the light scattered by the phonons is circularly polarized. The meticulous data presented in Figures (4) and (5) of their paper by the authors could hardly be doubted. The question that concerns me is regarding their claim, 'that phonons created by the conversion of magnons do carry spin'.…”

“…In this note we discuss the recent experiment [1] and argue that the measurements show that phonons exchange angular momentum with light but it is not spin of the phonons. We emphasize that this distinction is not just semantic [3] but of fundamental nature [4].…”

“…Photon is a vector particle with rest mass zero and spin one having only two projections -better understood in terms of helicity. Photon as a quantized electromagnetic radiation field continues to have fundamental questions: gauge-invariance, transversality and Lorentz covariance are controversial and unsettled issues, see references cited in [4] and also [7,8].…”