Controlled nucleation of point defects on a disclination line near a free surface during smectic-A-to-nematic directional melting

A disclination line populated with point defects that break the translational symmetry forms near a free nematic (N) interface in a confined geometry. The disclination line is, however, absent in the smectic-A phase (SmA). We use this fact to control the formation of point defect distributions on a disclination line by directional melting of the SmA phase in a temperature gradient. A threshold velocity (nu(th)) exists below which a defect-free disclination line is formed. The frequency of nucleation of point defects increases steadily for nu > nu(th), and exhibits a remarkable regularity. We derive an empirical scaling for nu(th) in terms of the experimental tuning parameters. We propose a simple model that allows to understand the formation of the point defects.