Circular phonon dichroism in d-wave altermagnets

Altermagnets, a new class of collinear antiferromagnets, exhibit momentum-dependent spin splitting and offer compelling advantages for antiferromagnetic spintronics. However, the magnetic order is intrinsically difficult to read out, which hinders practical applications. We propose finite-momentum circular phonon dichroism as a direct probe that distinguishes the two opposite out-of-plane Néel vector configurations in two-dimensional d-wave altermagnets. Combining Onsager reciprocity with C_2z lattice symmetry, we find that the dichroic signal reverses sign when the Néel vector is flipped for the in-plane phonon wave vectors. Moreover, a channel-resolved decomposition identifies the circular phonon dichroism originates from the interband coherent transitions. Representative finite-momentum cuts show pronounced dichroic asymmetric ratio, with |η_CPD| = 36%. Our work reveals that the circular-ultrasound absorption can distinguish the two opposite out-of-plane Néel vector configurations of d-wave altermagnets.