Condensed Matter Seminars

Colloids have provided versatile model systems to investigate rudimentary characteristics of material phases and have been employed as elementary units for spontaneous and directed self-assembly, mimicking atoms. Their size observable in the microscopy is feasible to track individual positions and orientations, so many phenomena including melting and nucleation have been studied at the single-particle level for better understanding atomic scale dynamics. In the same manner, we used Janus colloids to imitate spins in the observable scale under the microscope. We proved the phase transition of Janus spheres’ spin (orientation) order in Janus colloidal crystal, where the spin interaction is controlled by the external electric field. The spin configuration evolves from a random pattern to vortex and zigzag patterns, as increased the electric field. During this process, we measured the spin arrangement is changed from the short-range order to the quasi-long-range order through the power law decay of spatial correlation functions. Furthermore, we found the density of topological defects, which are vortex and anti-vortex, is correlated with the spin phase transition that is corroborated with the susceptibility of the spin order parameter. To describe the spin phase transition, we suggested the 2D Heisenberg model. Therefore, we expect that the designed system provides a platform to help understand spin behaviors in the single-particle level as well as plenty of phenomena induced by orientational interaction in atomic and molecular materials.