Blue Phase III: Topological Fluid of Skyrmions

Abstract

Skyrmions are topologically protected, vortexlike formations of a field that cannot be removed by any smooth transformation and emerge in a range of fundamentally different, either quantum or classical systems, from spin textures to chiral ferromagnets and chiral complex fluids. Notably, they are generally observed in thin ordered or disordered quasi-2D layers, but little is known about their three-dimensional structuring and organization, including structural transitions from 2D to 3D. Here, we show experimentally and numerically that the blue phase (BP) III of a chiral liquid crystal is a 3D fluid of chiral skyrmion filaments of the nematic orientational field, entangled with a 3D network of topological defect lines. It is an effective 3D dynamic fluid determined by the thermal fluctuations of two distinct branches of excitations: rapid internal fluctuations of the skyrmion structure and a slow collective motion of the skyrmion filaments. When confined to less than an approximately 150-nm layer, the 3D bulk skyrmion fluid transforms into a different effectively 2D liquid of half-skyrmions, with the dynamics of the skyrmion liquid slowing down by an order of magnitude and with the individual skyrmions lingering, and even disappearing into, and reappearing from the homogeneous liquid crystal. The thickness-temperature phase diagram actually shows that both the BPIII and BPI phases are made of skyrmions, which when confined to less than approximately 150 nm cells transform equally into a 2D half-skyrmion liquid. The temperature range of this 2D half-skyrmion liquid is much broader than the temperature interval of BP phases, which makes BP materials interesting for broad-temperature-range skyrmionic applications. We envisage a soft matter skyrmionic device, in which skyrmions are created and detected by light.