Chiral Phonons with Giant Magnetic Moments in a Topological Crystalline Insulator

We have studied the magnetic response of transverse optical phonons in Pb${}_{1-x}$Sn${}_x$Te films. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating that these phonon modes become chiral in magnetic fields. Films in the topological crystalline insulator phase ($x>0.32$) exhibited magnetic moment values that are larger than those for topologically trivial films ($x<0.32$) by two orders of magnitude. Furthermore, the sign of the effective $g$-factor was opposite in the two phases, which can be explained by our theoretical model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of electronic band structure.