Nematic twist-bend phase under external constraints

摘要

The recently discovered twist-bend nematic phase, N-TB, has short-pitched heliconical structure with doubly degenerate handedness. In contrast to the classic nematic, in the N-TB phase the director is spontaneously distorted, resulting in unusual elastic properties. The response of the N-TB 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 N-TB behaviour is theoretically investigated under chiral doping and strong electric fields. We show that the chiral doping removes the N-TB degeneration and modifies the conical tilt angle, leaving the pitch unchanged. Thus, the N-TB 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 N-TB 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.