Three dimensional convective instability in nematic liquid crystal under an ac field

Using a light scattering technique, fluctuations in nematic MBBA under an a.c. field are investigated near a three dimensional convective instability point. It is found that the instability is accompanied by a critical slowing down of the thermally excited twist fluctuation and the appearance of the macroscopic undamped oscillation. A distribution of the molecular orientation is also proposed.