Forbidden walls

In this paper the presence of periodic twist-bend walls in the upper vicini ty df the Freedericksz critical point of a nematic calamitic sample is repo rted. According to the existing theory, the formation of these structures c ould not happen in this region of the magnetic field. The length of the per iodicity of these walls as well as the time spent with their formation have been measured, and it was demonstrated that the coherent motion of the nem atic material cannot drive their construction. We assume that the mechanism responsible for their appearance results from the magnification of some se lected random fluctuations occurring at the neighborhood of the Freedericks z critical point. [S1063-651X(99)02909-S].