Emergent elastic fields induced by topological phase transitions: Impact of molecular chirality and steric anisotropy

Topological phase transitions into skyrmion and half-skyrmion (meron) phases are widely observed in condensed matter, such as chiral magnets and liquid crystals. They are utilized to design magnetoelectric, optical, and mechanoresponsive materials by controlling such topological phases. However, the role of the elastic field in nonuniform topological phases is elusive, though the essential role of crystal elasticity in uniform ordered crystal phase has been recognized. To elucidate this problem, we construct a model describing chiral molecules and colloids in quasi-two-dimensional molecular crystals, which incorporates intermolecular chiral twisting and spheroidal steric interactions. We reveal that emergence of the elastic fields from the competition between steric anisotropy and intermolecular twisting is a key to control uniform, helical, and half-skyrmion structures. By utilizing the coupling between the spheroidal orientations and the elastic fields, these topological phases are switched by temperature, external electromagnetic fields, and anisotropic stresses, where a re-entrant phase transition between the helical and the half-skyrmion phases is discovered. Our results imply that controlling the emergent elastic fields is crucial for obtaining a fundamental physical principle for controlling topological phases using chiral molecular and colloidal crystals.