Helically modulated electroclinical effects in twist grain boundary liquidcrystals

We report the first observation of an electroclinical effect at the TGB(C)- TGB(A) transition induced by an external DC electric field applied perpendi cular to the pitch direction. Upon increasing the field, the smectic layers rather than the director field tilt over relative to the helical axis, all owing to detect the effect by X-ray scattering from well aligned samples. T he observations are qualitatively interpreted in the frame of a mean field phenomenological model of a helically modulated electroclinical effect.