Director field in plastic crystals
The director field in a plastic crystal is calculated by particle-resolved Monte Carlo computer simulations of two-dimensional, slightly anisometric hard spherocylinders exposed to an external periodic substrate potential. We investigate the structure of the director field in the Wigner-Seitz cell and find a topological defect structure that can be controlled with the substrate potential. At zero potential we find a charge −1/2 defect at the corners besides the expected defect in the centre with unit topological charge. When switching the substrate potential on, the corner defects are surrounded by three satellite defects which bear charge −1/2, too. Additionally, we then find two charge +1/2 defects on each edge of the unit cell. Finally, within a simplified model, we obtain a qualitative explanation for this defect structure. Our predictions can in principle be verified by using particle-resolved experiments of colloidal plastic crystals.