Phonon chirality and Hall effect in Janus materials

Janus materials provide an intrinsic platform for inversion-symmetry breaking and chirality-related phonon phenomena. Here, based on first-principles calculations, we investigate the impact of out-of-plane asymmetry on chiral phonons in Janus SbTeI. We show that exchanging the upper and lower atomic layers produces distinct chirality responses in different structural phases. In the 2H-phase, the phonon chirality in the in-plane directions is reversed while the out-of-plane component is preserved, whereas in the 1T-phase, all three chirality components are simultaneously flipped. We point out that the distinct responses are fundamentally determined by the mirror symmetry Mz and the spatial inversion symmetry P in different phases. Furthermore, these chirality reversals lead to corresponding changes in phonon angular momentum, magnetization under a temperature gradient, and the phonon Hall response in the presence of a magnetic field. Our findings highlight the essential role of Janus asymmetry in phonon chirality and related transport phenomena.