Geometric transformation and three-dimensional hopping of Hopf solitons

Abstract: 3D topological solitons are marvels of mathematical physics that arise in theoretical models in elementary particle and nuclear physics, condensed matter, and cosmology. A particularly interesting type of them is described by the mathematical Hopf map from a hypersphere to an ordinary sphere, which in the physical 3D space exhibits inter-linked circle-like or knotted localized regions of constant order parameter values. Despite their prevalence in models, such solitons remained experimentally elusive until recently, when hopfions were discovered in colloids and chiral liquid crystals, whereas the so-called "heliknotons" were found both individually and within triclinic 3D lattices while smoothly embedded in a helical background of chiral liquid crystals. Constrained by mathematical theorems, stability of these 3D excitations is thought to rely on a delicate interplay of competing free energy contributions, requiring applied fields or confinement. Here we describe such 3D solitons in a material system where no applied fields or confinement are required. Nevertheless, electric fields allow for inter-transforming heliknotons and hopfions to each other, as well as for 3D hopping-like dynamics arising from nonreciprocal evolution of the molecular alignment field in response to electric pulses. Stability of these solitons both in and out of equilibrium can be enhanced by tuning anisotropy of parameters that describe energetic costs of gradient components in the field, which is implemented through varying chemical composition of liquid crystal mixtures. Numerical modelling reproduces fine details of both the equilibrium structure and Hof-index-preserving out-of-equilibrium evolution of the molecular alignment field during switching and motions. Our findings may enable myriads of solitonic condensed matter phases and active matter systems, as well as their technological applications.