Symmetry-Enforced Chiral Phonons in Altermagnets via Magnon-Phonon Coupling

Chiral phonons are attractive for spintronics applications, however, their zero-field generation in conventional antiferromagnets is forbidden by combined parity and time-reversal ($\mathcal{PT}$) symmetry. Here we demonstrate the emergence of chiral phonons in $\mathcal{PT}$-breaking altermagnetic systems at zero field arising from relativistic magnon-phonon coupling. Focusing on the prototypical altermagnet CrSb, we utilize first-principles methods to calculate the hybridized magnon-polarons across the complete Brillouin zone. We show that this coupling imprints an altermagnetic $g$-wave symmetry directly onto the phonon angular momentum. Furthermore, we demonstrate anomalous spin and phonon angular momentum Nernst responses arising from finite Berry curvatures. These findings establish that chiral lattice dynamics can arise in compensated magnetic ground states without requiring external fields, positioning bulk altermagnets as material candidates for zero-field spin caloritronics and chiral phononics.