Noise-induced threshold shift and pattern formation in electroconvection by controlling characteristic time scales.

We report external noise-induced threshold shift and pattern formation in ac-driven electroconvection (EC) in nematic liquid crystals. We investigate the noise intensity dependence of the threshold and determine the relationship f(c)(*)=hf(cd)(a) (α ∼ 1.4, h ∼ 0.1) between the characteristic cutoff frequency (f(c)(*)) of the noise and the characteristic ac frequency (f(cd)) of EC. For f(c)>f(c)(*), the noise contributes to stabilizing EC (i.e., upward threshold shift), whereas for f(c)<f(c)(*), it contributes to destabilizing EC (i.e., downward threshold shift). For f(c)=f(c)(*), the noise makes no contribution to the threshold shift. However, the characteristic wavelength of EC strongly depends on the noise intensity but not on f(c).