Two-step crystallisation in a 2D active magnetic granular system confined by a parabolic potential

We studied the two-step crystallisation process of a magnetic active 2D-granular system placed on different lens concaveness and under the action of an alternating magnetic field which controls its effective temperature. We have observed that the two-step features of the crystallisation process are more evident as the depth of the parabolic potential increases. At the initial formation of the nucleus, as a first step, in the central region of the lens an amorphous aggregate is formed. In an ulterior second step, this disordered aggregate, due to the effective temperature and the perturbations caused by the impacts of free particles moving in the surrounding region, evolves to an ordered crystalline structure. The nucleus size is larger for deeper concaveness of the parabolic potential. However, if the depth of the parabolic potential exceeds a certain value, the reordering process of the second step does not occur. The crystal growth occurs similarly; small disordered groups of particles join the nucleus, forming an amorphous shell of particles which experiments a rearranging while the aggregate grows. In the explored range of the depths of the parabolic potential, crystallisation generally occurs quicker as the deeper parabolic potential is. Also, aggregates are more clearly round-shaped as parabolic potential depth increases. On the contrary, the structures are more branched for a smaller depth of the parabolic potential. We studied the structural changes and features in the system by using the sixth orientational order parameter and the packing fraction.

Automated summary

In this study, we investigated the two-step crystallization process of a magnetic active 2D-granular system placed on different lens concaveness and under the action of an alternating magnetic field. We found that the depth of the parabolic potential plays a significant role in the two-step features of the crystallization process. The initial formation of an amorphous aggregate in the central region of the lens is followed by the evolution to an ordered crystalline structure due to effective temperature and perturbations caused by free particles. The size of the nucleus and the shape of the aggregates are influenced by the depth of the parabolic potential.