Diffusionless relaxation of half-skyrmion liquid, hexatic, and crystalline states in a chiral molecular crystal

Abstract

Particles in a crowded environment exhibit slow anomalous diffusion, and their efficient manipulation is important in controlling transport phenomena in complex materials. Skyrmions and half-skyrmions, spatially localized quasiparticles observed in magnetic systems and liquid crystals, also exhibit diffusive motion. They exhibit normal diffusion in dilute conditions. However, the cooperative dynamics and diffusion of skyrmions and half-skyrmions in their condensed liquid, hexatic, and crystalline phases are elusive. Here we show in the half-skyrmion condensed phases that the fusion and fission of half-skyrmions, not their diffusion, are responsible for the primary structural relaxation. The fusion and fission occur due to the non-conserved nature of the quasiparticle number density. The diffusion, which contributes to the secondary structural relaxation, is suppressed by cages formed by surrounding half-skyrmions, whereas enhanced by Mermin-Wagner fluctuation characteristic to two-dimensional systems, leading to subdiffusive motion. Large displacement of half-skyrmions is locally excited by fusion-fission and bond-breaking between adjacent half-skyrmions via heterogeneous elastic fields. Furthermore, the motion of half-skyrmions couples with transverse and longitudinal sound wave excitation differently, where the transverse sound wave is more attenuated than the longitudinal one due to the coupling between transverse sound wave and half-skyrmion deformation. We also discuss the {relationship} between half-skyrmion diffusion in our system and skyrmion diffusion in magnetic systems. Our result provides a qualitative difference in dynamical properties between half-skyrmion gaseous and condensed phases, suggesting the efficient manipulation of high-density half-skyrmions and skyrmions.