| Soft-mode turbulence (SMT) observed in electroconvection of homeotropic nematic systems is a kind of spatiotemporal chaos induced by nonlinear interaction between the Nambu-Goldstone modes and the convective modes. In the SMT, we found a new topological defect called blackline. We are interested to investigate the blackline which appears as a line defect, since the SMT can be regarded to 2D XY system. By detailed observations and analysis, however, it is clarified that the blackline is a structure of the nematic director in the x-y plane and includes a sequence of point defects. The existence of the blackline is then discovered as a consequence of the symmetry of the nonlinear interaction between the two modes. We measured the density of the blackline as a function of control parameters, namely ac voltage and frequency. Finally, we discussed the similarity between the density of the blackline and that of the point defect in the conventional 2D XY model. |
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