Nematic twist-bend phase under external constraints

Abstract

ABSTRACTThe recently discovered twist-bend nematic phase, NTB, has short-pitched heliconical structure with doubly degenerate handedness. In contrast to the classic nematic, in the NTB phase the director is spontaneously distorted, resulting in unusual elastic properties. The response of the NTB phase to external stimuli, like chiral doping or applied fields might provide further information about its structure and can find utilisation in practical applications. Here, the NTB behaviour is theoretically investigated under chiral doping and strong electric fields. We show that the chiral doping removes the NTB degeneration and modifies the conical tilt angle, leaving the pitch unchanged. Thus, the NTB helical twisting power is very high and strongly non-linear. Under electric field, we consider separately the ferroelectric, flexoelectric and dielectric couplings. We show that the experiments reported so far disagree with the ferroelectric behaviour, indicating that the NTB phase is not spontaneously polarised. On the contrary, the observed polar effects fit well with the flexoelectric coupling, confirming the degenerated heliconical structure of the phase. Under very strong fields, we predict a second-order twist-bend nematic – nematic phase transition due to the dielectric torque on the director.