Search

Representation of the oblique helicoidal structure with a nanometric pitch of typical of the twist-bend nematic phase. A variational approach is employed to search for exact solutions describing the director profile in the framework of an extended Frank elastic energy density. This extension of the energy density of the usual nematic phase adds an extra element of symmetry represented by the axis, thus augmenting the number of possible phases described by the elastic energy, which includes modulated phases. The solutions corresponding to these phases are searched by exploring the symmetry properties and the existence of conserved quantities, which permits us to reduce the general solution of the variational problem to quadratures. This approach may be particularly useful to investigate the existence and stability of the recently discovered twist-bend nematic phase as well as to predict the orientational properties of other periodically modulated phases. [ABSTRACT FROM PUBLISHER]

The twist-bend nematic (NTB) phase is a doubly degenerated heliconical structure with nanometric pitch and spontaneous bend and twist deformations. It is favored by symmetry-breaking molecular structures, such as bent dimers and bent-core molecules, and it is currently one of the burgeoning fields of liquid-crystal research. Although tremendous advances have been reported in the past five years, especially in molecular synthesis, most of its potential applications are held back by the lack of a proper and definitive elastic model to describe its behavior under various situations such as confinement and applied field. In this work we use a recently proposed stable state elastic model and the fact that the mesophase behaves as a lamellar structure to propose a mesoscopic or coarse-grained model for the NTB phase. By means of standard procedures used for smectic and cholesteric liquid crystals, we arrive at a closed-form energy for the phase and apply it to a few situations of interest. The predicted compressibility for several values of the cone angle and the critical field for field-induced deformation agree well with recent experimental data. [ABSTRACT FROM AUTHOR]

The twist-bend nematic phase, NTB, may be viewed as a heliconical molecular arrangement in which the director n precesses uniformly about an extra director field, t. It corresponds to a nematic ground state exhibiting nanoscale periodic modulation. To demonstrate the stability of this phase from the elastic point of view, a natural extension of the Frank elastic energy density is proposed. The elastic energy density is built in terms of the elements of symmetry of the new phase in which intervene the components of these director fields together with the usual Cartesian tensors. It is shown that the ground state corresponds to a deformed state for which K22>K33. In the framework of the model, the phase transition between the usual and the twist-bend nematic phase is of second order with a finite wave vector. The model does not require a negative K33 in agreement with recent experimental data that yield K33>0. A threshold is predicted for the molecular twist power below which no transition to a twist-bend nematic may occur. [ABSTRACT FROM AUTHOR]

From the elastic point of view, the twist-bend nematic phase, (NTB), has a ground state characterised by a spontaneous bend with a precession of the directornalong the direction of an axist. The tilt angle θ formed between the directornand the axistgives rise to a heliconical structure with a nanometric helical pitchp. Using an elastic continuum theory recently proposed as an extension of the Frank energy for these materials, we investigate the influence of an external magnetic field on the pitch of the NTBphase. For a material with positive magnetic susceptibility anisotropy, when the magnetic fieldHis perpendicular tot, it is shown that the pitch of the nematic directornprecession increases withH, and at a critical value ofit becomes infinite as in a cholesteric liquid crystal, that is, one gets a uniform orientation ofn. Our results include the behaviour of a cholesteric under field as a limiting case, and are obtained in the simple case where θ is assumed as uniform throughout the sample. [ABSTRACT FROM AUTHOR]

The recently discovered twist-bend nematic phase, Ntb, is a non-uniform equilibrium nematic phase that presents a spontaneous bend with a precession of the nematic director,n, on a conical helix with a tilt angleθand helical pitchP. The stability of the Ntbphase has been recently demonstrated from the elastic point of view by extending the Frank elastic energy density of the nematic phase to include the symmetry element of the helical axis,t. In the present article, we investigate the influence of an external bulk field (magnetic or electric) on the Ntbphase. Using symmetry arguments we derive the expression for the flexoelectric polarisation in twist-bend nematic phases. We show that, besides the standard contribution related to the spatial variation of the nematic director, two new contributions connected with the existence of the helical axis appear. In the ground state, where the nematic deformation is a pure heliconical deformation, the new contribution vanishes identically, and the total flexoelectric polarisation is perpendicular to the nematic director. Furthermore, as an example, we study the role of an external magnetic field applied parallel to the helical axis for a material with positive magnetic susceptibility anisotropy. We show that the field modifies the range of values of the coupling parameter between the director and the helical axis, thus shifting the interval of values for which this coupling results in the Ntbphase. [ABSTRACT FROM PUBLISHER]