Emergent elasticity linked to topological phase transitions controlled via molecular chirality and steric anisotropy

Self-organisation into spatially modulated structures has different nature from phase transition into uniform states. Skyrmions and half-skyrmions (merons) are representatives of such structures and are utilised in designing magnetoelectric, optical, and mechanoresponsive materials by controlling topological phases. However, skyrmions and half-skyrmions in molecular solids are rarely studied, though there is a universality in theoretical descriptions between magnetic and molecular systems with chiral interactions. Here we develop a simple physical system for controlling topological phases in a solid with chirality. We reveal that emergence of elastic fields from anisotropic steric interactions and intermolecular twisting is a key to control helical and half-skyrmion structures. Utilising the coupling between the emergent elastic fields and molecular orientations, we successfully control topological phases by temperature, external electromagnetic fields, and anisotropic stresses. The concept of the emergent elasticity provides a control system for designing molecular and macromolecular solids with tunable electro- and magneto-mechanical properties.