Electric-field-induced magnetochiral dichroism in a ferroaxial crystal NiTiO₃

NiTiO₃ is known as a representative ferroaxial material, which exhibits a spontaneous and uniform rotational structural distortion. Such a rotational distortion is characterized by the breaking of a mirror symmetry whose mirror plane includes the rotation axis. In contrast to chirality in which any mirror symmetry is forbidden, a mirror symmetry with the mirror plane perpendicular to the rotation axis is allowed in a ferroaxial material. In other words, chirality can be induced in a ferroaxial material by breaking the remaining mirror symmetry by applying external fields. In NiTiO₃, optical activity induced by an electric field, called electrogyration, has been observed, which is regarded as the effect owing to electric field induced chirality. In this work, we focus on another effect characteristic of chiral materials, magnetochiral dichroism (MChD). MChD is asymmetry in optical absorption between two counter propagating light beams in a magnetized chiral material. In NiTiO_3, electric-field-induced MChD is expected to be observed when electric and magnetic fields are simultaneously applied. We measured transmittance spectra of NiTiO₃ under simultaneous applications of electric and magnetic fields along the rotation axis (the c axis). As a result, we observed finite directional dichroism whose signal is proportional to both applied electric and magnetic fields. This result is nothing less than electric-field-induced MChD in a ferroaxial material.