Spin vortices and antivortices of Janus colloidal crystals

Janus colloidal particles have spin vectors normal to their Janus equator and resulting spin-orientational order that differs from the orientational order considered in traditional models of hexatic melting. Based on imaging their monolayers experimentally, we describe their topological spin defects and show that the Berezinskii-Kosterlitz-Thouless model describes their orientational order when spin orientations are driven into two dimensions by an external AC electric field. This is nontrivial, as the model assumes isotropic nearest-neighbor interactions, which is not the case for this system. These experiments reveal the predictive power of this idealized model beyond the strict validity of its assumptions.