Directional and circular dichroism of ferro-rotational phonons in MnTiO₃

Chirality, which describes the asymmetry between a structure and its mirror image, is ubiquitous in nature and dictates various chiral phenomena, such as natural optical activity, chiral phonons, and chiral magnons. Planar chirality arises from the breaking of reflection symmetry about two orthogonal mirrors, rather than three, making it typically centrosymmetric—distinct from the non-centrosymmetry of conventional chirality. Using resonant inelastic X-ray scattering with circularly polarized X-rays, we identified circularly polarized phonons in ferro-rotational MnTiO₃, referred to as ferro-rotational phonons. These phonons exhibit clear signatures of non-reciprocal directional and circular dichroism, meaning that their sign reverses when the incident direction of circular X-ray is flipped. Furthermore, this new type of two-dimensional chiral phonon features unusual orbital selectivity, involving hybridization exclusively between the O 2p states and the Ti t_2g states.