Twist-bend nematic liquid crystals in high magnetic fields

Abstract

We present magneto-optic measurements on two materials that form the recently discovered twist-bend nematic (N_{tb}) phase. This intriguing state of matter represents a fluid phase that is orientationally anisotropic in three directions and also exhibits translational order with periodicity several times larger than the molecular size. N_{tb} materials may also spontaneously form a visible, macroscopic stripe texture. We show that the optical stripe texture can be persistently inhibited by a magnetic field, and a 25T external magnetic field depresses the N-N_{tb} phase transition temperature by almost 1{∘}C. We propose a quantitative mechanism to account for this shift and suggest a Helfrich-Hurault-type mechanism for the optical stripe formation.

Hierarchy of Periodic Patterns in the Twist-bend Nematic Phase of Mesogenic Dimers

“…These can lead to a variety of applications including nano-and micro patterning, colloid and particle positioning and manipulation and guided self-assembly of complicated structure. For instance, the sub-micron "layered" pattern reported here may correspond to the pseudo-layers proposed by the Kent group for explaining the self-deformation stripes by Helfrich-Hurault-type mechanism [15]. Since these self-deformation patterns depend on the macroscopic parameters of the sample (such as the confining surface size and alignment), varying these parameters should provide for a rich experimental space.…”

Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane: A dielectric,H2NMR, and calorimetric study

“…2(b)]. 267 The stripes grow slowly parallel to the surface easy axis with a 268 periodicity of the order of twice the cell thickness as reported 269 for other twist-bend nematic forming dimers [1,17]. For slow 270 cooling rates, the stripes are totally developed showing tilted 271 bands across them, as in a ropelike texture, characteristic of a 272 twist-bend nematic phase [2], while for faster cooling rates a 273 less uniform texture with regions of focal conics is observed.…”

“…This study has been followed by intense 72 research activity focused on the structure and properties of the 73 N TB phase. Different authors have described a characteristic 74 ropelike texture with stripes parallel to the alignment axis 75 for thin films of planar aligned samples [1,2,9,16,17]. With 76 the sample between a cover slip and microscope slide, 77 focal-conic-defect and parabolic-defect textures have been 78 The chiral structure associated with the heliconical twist-bend 126 nematic phase is established by 2 H NMR experiments using calorimetry experiments we present a detailed description 131 of the N -I and N TB -N phase transitions.…”

Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

“…3(c)], and in the form of propagating focal-conic domains (FCDs), such as is usually observed in smectic liquid crystals with negative dielectric anisotropy [24,25]. The "pseudolayered" nature of the heliconical structure [15] is reflected in the gradual relaxation of the FCDs to homeotropic alignment after removal of the field. As Fig.…”

“…Within this approximation, which has previously been used to explain the effect of high magnetic fields on the N TB phase [15] and to account for its flow properties [16], the normal fluctuation modes involving the director may be mapped onto those of a chiral smectic-A phase, with effective layer spacing equal to the pitch, effective director parallel to the local pitch axis, and effective elastic constants that arise from the short-pitch orientational modulation rather than from a true mass density wave.…”