arXiv subjects

Vassili G. Nazarenko

Publications and source records attributed to Vassili G. Nazarenko.

5 recordsLinked to original sources

arXiv2025

Polar and apolar light-induced alignment of ferroelectric nematic at photosensitive polymer substrate

Surface alignment of a recently discovered ferroelectric nematic liquid crystal (NF) is usually achieved by buffed polymer films, which produce a unidirectional polar alignment of the spontaneous electric polarization. We demonstrate that photosensitive polymer substrates could provide a wider variety of alignment modes. Namely, a polyvinyl cinnamate polymer film irradiated by linearly polarized ultraviolet (UV) light yields two modes of surface orientation of the NF polarization. (1) A planar apolar mode, in which the equilibrium NF polarization aligns perpendicularly to the polarization of normally impinging UV light; the NF polarization adopts either of the two antiparallel states. (2) A planar polar mode, produced by an additional irradiation with obliquely impinging UV light. In this mode, there is only one stable azimuthal direction of polarization in the plane of the substrate. The two modes differ in their response to an electric field. In the planar apolar mode, polarization can be switched between two states of equal surface energy. In the planar polar mode, the field-perturbed polarization relaxes back to the single photoinduced easy axis once the field is switched off. The versatility of modes and absence of mechanical contact make the photoalignment of the NF attractive for practical applications.

cond-mat.soft↗

arXiv2024

Periodic splay Fréedericksz transitions in a ferroelectric nematic

Electric field-induced splay of molecular orientation, called the Fréedericksz transition, is a fundamental electro-optic phenomenon in nonpolar nematic liquid crystals. In a ferroelectric nematic NF with a spontaneous electric polarization P, the splay is suppressed since it produces bound electric charges. Here, we demonstrate that an alternating current (ac) electric field causes three patterns of NF polarization. At low voltages, P oscillates around the field-free orientation with no stationary deformations. As the voltage increases, the polarization acquires stationary distortions, first splay and twist in a stripe pattern and then splay and bend in a square lattice of +1 and -1 defects. In all patterns, P oscillates around the stationary orientations. The stationary bound charge is reduced by a geometrical splay cancellation mechanism that does not require free ions: the charge created by splay in one plane is reduced by splay of an opposite sign in the orthogonal plane.

cond-mat.soft↗

arXiv2024

Conic sections in ferroelectric nematics: experiments and mathematical modeling

Domain structure of a fluid ferroelectric nematic is dramatically different from the domain structure of solid ferroelectrics since it is not restricted by rectilinear crystallographic axes and planar surface facets. We demonstrate that thin films of a ferroelectric nematic seeded by colloidal inclusions produce domain walls in the shape of conics such as a parabola. These conics reduce the bound charge within the domains and at the domain walls. An adequate description of the domain structures requires one to analyze the electrostatic energy, which is a challenging task. Instead, we demonstrate that a good approximation to the experimentally observed polydomain textures is obtained when the divergence of spontaneous polarization - which causes the bound charge is heavily penalized by assuming that the elastic constant of splay in the Oseen-Frank energy is much larger than those for twist and bend. The model takes advantage of the fact that the polarization vector is essentially parallel to the nematic director throughout the sample.

cond-mat.soft↗

arXiv2024

Polar self-organization of ferroelectric nematic liquid crystal molecules on atomically flat Au(111) surface

Understanding nanoscale mechanisms responsible for the recently discovered ferroelectric nematics can be helped by direct visualization of self-assembly of strongly polar molecules. Here we report on scanning tunneling microscopy (STM) studies of monomolecular layers of a ferroelectric nematic liquid crystal on a reconstructed Au(111) surface. The monolayers are obtained by deposition from a solution at ambient conditions. The adsorbed ferroelectric nematic molecules self-assemble into regular rows with tilted orientation, resembling a layered structure of a smectic C. Remarkably, each molecular dipole in this architecture is oriented along the same direction giving rise to polar ferroelectric ordering.

cond-mat.soft↗

arXiv2013

Effect of collective molecular reorientations on Brownian motion of colloids in nematic liquid crystal

In the simplest realization of Brownian motion, a colloidal sphere moves randomly in an isotropic fluid; its mean squared displacement (MSD) grows linearly with time $\textitτ$. Brownian motion in an orientationally ordered fluid, a nematic, is anisotropic, with the MSD being larger along the axis of molecular orientation, called the director. We show that at short time scales, the anisotropic diffusion in a nematic becomes also anomalous, with the MSD growing slower (subdiffusion) and faster (superdiffusion) than $\textitτ$. The anomalous diffusion occurs at time scales that correspond to the relaxation times of director deformations around the sphere. Once the nematic melts, the diffusion becomes normal and isotropic. The experiment shows that the deformations and fluctuations of long-range orientational order profoundly influence diffusive regimes.

cond-mat.soft↗