Superfast domain wall motion in lithium niobate single crystals. Analogy with crystal growth

摘要

In this paper, we studied the superfast domain walls that appear after domain merging during polarization reversal in congruent lithium niobate single crystals by in situ optical visualization with a high temporal resolution. The superfast domain walls were observed during the fast transformation of the domain shape from concave polygonal that appears after merging to the convex polygonal one with slow domain walls only. Orientations and velocities of superfast domain walls were measured during transformation of an artificial polygonal X-shaped domain (with all concave angles appearing after merging) to a hexagonal one. The convex growth of isolated hexagonal domains was governed by the slowest domain walls, while the concave growth after domain mergingby the superfast walls with three orders of magnitude higher velocity. We reconstructed experimentally the v-plot (kinetic Wulff plot) for domain wall motion by analysis of both convex and concave domain growth. The slowest walls (Y-walls) correspond to the v-plot deep cusps having six-fold symmetry, whereas the superfast walls correspond to the v-plot maxima having three-fold symmetry with a difference between Y+ and Y- crystallographic directions. The field dependences of the velocities of the superfast domain walls followed the activation law, taking into account the existence of the bias field.