Formation scenarios for nonlinear patterns in electroconvection under controlling goldstone modes in magnetic field

We report novel scenarios for the transitions from a stationary periodic state to spatio-temporal chaos (STC) for electroconvection in homeotropically aligned nematics, in which a constant magnetic field H is applied perpendicularly to an electric field. Above a characteristic frequency f_L (in the normal roll regime), when increasing the applied electric field, stationary periodic roll patterns become unstable, and evolve to STC at a certain electric field. For f > f_L we find the second stationary periodic roll pattern (i.e., abnormal rolls) at higher electric fields for H > H*, where H* is a characteristic magnetic intensity. At much higher electric field the reentrant stationary periodic rolls evolve to STC again. Reentrant stationary rolls are not observed for H < H*. Below f_L (in the oblique roll regime), on the other hand, with increasing the electric field, stationary periodic rolls also become unstable toward STC. In the oblique roll regime, however, we find no reentrant stationary rolls below and above H*. The abnormal roll instability responsible for the reentrant rolls corresponds to a homogeneous azimuthal rotation of the director and plays important role for the novel scenarios. Based on systematic studies, we have determined phase diagrams in the H - ε plane, where ε is a normalized electric voltage.