Fluctuation modes in the twist bend nematic phase of certain liquid crystal mixtures studied by dynamic light scattering*

The ``twist-bend nematic''($N_{TB}$) is a recently discovered phase of liquid crystals. Its ground state features a heliconical molecular arrangement in which the nematic director precesses uniformly about a fixed axis, at a finite angle to this axis. The helicoid has a nanoscale pitch. We present the dynamic light scattering studies in two different mixtures of a dimeric material (M2), which exhibits nematic and $N_{TB}$ phases. In the nematic phase, two fluctuation modes are observed: one is the usual hydrodynamic, uniaxial director mode, while the second is clearly non-hydrodynamic and corresponds to fluctuations of biaxial order. The non-hydrodynamic mode is also observed in the $N_{TB}$ phase, with a significantly higher and strongly temperature-dependent relaxation rate. Scattering from the twist-bend director mode disappears in the $N_{TB}$ phase, while splay fluctuations still contribute. We discuss our results in terms of a theoretical model based on a combination of two elements: (1)a coarse-grained elastic free energy (in which the $N_{TB}$ is modeled a smectic-like, pseudo-layered structure) and (2)a recent theory [1] describing local polar order in the $N_{TB}$ phase.\\[4pt] [1] Phys.Rev.E 87,052503(2013)