Chiral Phonons Arising From Chirality-Selective Magnon-Phonon Coupling

Abstract

Chiral phonons are desirable for applications in spintronics but their generation and control remains a challenge.Here we demonstrate the emergence of truly chiral phonons from selective magnon-phonon coupling in inversion-symmetric magnetic systems. Considering bcc Fe as example, we quantitatively calculate hybridized magnon-phonon quasiparticle states across the entire Brillouin zone utilizing first-principles calculations. Our findings challenge conventional magneto-elastic interpretations and reveal finite zero-point phonon angular momentum and strong anomalous thermal Hall responses linked to finite (spin) Berry curvatures. Our results further establish that the existence of chiral phonons, particularly along high-symmetry directions, is common in many magnetic materials, offering promising avenues for novel spintronic and phononic devices.

Figures (4)

• Figure 1: Coupled magnon-phonon bands in bcc Fe calculated along various high-symmetry paths of the BZ starting from $\Gamma=(0,0,0)$. Labels LA, TA, and mag indicate the predominant character of the mode far away from the avoided crossings and the colors encode the phonon chirality $\Sigma = {\bm{L}} \cdot {\bm{v}}/|{\bm{v}}|$, with ${\bm{v}}=\partial\omega/\partial{\bm{k}}$ being the group velocity. The insets zoom in on the avoided crossings (indicated by the small grey rectangles), and the grey dashed lines are the bare magnon energies. Note that the bare TA modes are degenerate for $\Gamma \rightarrow (0,0,2\pi/a)$ and $\Gamma \rightarrow (\pi/a,\pi/a,\pi/a)$.
• Figure 2: Phonon angular momenta and chiralities of the coupled magnon-phonon bands computed for bcc Fe. Plots in each row illustrate (a) $L^x$, (b) $L^z$, (c) $\Sigma$ without applied magnetic field, and (d) $\Sigma$ with a field of $B=10T$ for the different magnon-phonon modes with label $n\in \{2,3,4\}$. The mode with $n=1$ (LA phonon) is achiral and hence not shown. $L^y$ is zero in the $k_x$-$k_z$ plane depicted here.
• Figure 3: Nonzero component $L_z$ of the equilibrium phonon angular momentum per unit cell for bcc Fe, shown in (a) versus temperature and for applied magnetic fields as labeled and in (b) versus rescaled SLC strength.
• Figure 4: Anomalous thermal Hall conductivity (a) and spin Nernst coefficient (b) due to magnon-polarons in an ultrathin Fe(001) film. The magnetization is in out-of-plane direction and a magnetic field with values as labeled is applied in the same direction.