arXiv subjects

Arjun Ghimire

Publications and source records attributed to Arjun Ghimire.

3 recordsLinked to original sources

arXiv2026

Landau theory applied to antiferroelectric ordering in ferroelectric nematic liquid crystals

The polarization and density modulation associated with antiferroelectric ordering is studied experimentally as a function of temperature in two ferroelectric nematic liquid crystals, the prototypical single compound (DIO) and a commercial mixture (FNLC919). The modulation wavenumber qA is determined by small angle X-ray diffraction from the weak smectic-like density wave (wavenumber qS = 2qA) that accompanies the polarization modulation. Results for qS and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the para-/antiferro-/feroelectric sequence of phase transitions in solid ferroelectrics. The analysis indicates that the polarization modulation is reasonably well approximated by a simple sinusoid in the antiferroelectric phase of DIO, whereas in FNLC919 the modulation develops a strongly soliton-like profile (with sharply decreasing wavenumber) close to the antiferro- to ferrolectric transition.

cond-mat.soft↗

arXiv2025

Ferroelectric Nematic and Smectic Liquid Crystals with Sub-Molecular Spatial Correlations

A number of highly polar three ring rod-shaped compounds with a terminal thiophene ring have been synthesized and the physical properties of a subset are reported in detail. On cooling from the isotropic fluid, they directly transition to a ferroelectric nematic liquid crystal (NF) phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20% larger than that of usual ferroelectric nematic liquid crystal materials. Such large polarization values are due to the ~20% larger mass densities of these thiophene compounds compared to most of the NF materials with short terminal chains. These unusual properties are consequences of tighter molecular packing due to the lack of flexible terminal chains. Below the NF phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity ~1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.

cond-mat.soft↗

arXiv2025

Director-layer dynamics in the antiferroelectric smectic-ZA phase of a ferroelectric nematic liquid crystal

A dynamic light scattering study of director-layer fluctuations in the antiferroelectric smectic-ZA phase of the ferroelectric nematic liquid crystal DIO is reported. The dynamics are consistent with the distinctive feature of the ZA phase that the smectic layers form parallel to the axis of molecular orientational order (director). A model is developed to describe quantitatively the dispersion of the fluctuation relaxation rates. The model is based on a specialization of the elastic free energy density of the smectic-C phase to the case of 90 degree director tilt, a "first-order" approximation of the viscous stresses by their form for an incompressible uniaxial fluid, and a treatment of the effect of chevron layer structure that develops in planar sample cells due to temperature-dependent layer shrinkage, as documented in previous studies on DIO. From the modeling, the layer compression elastic constant is estimated to be ~100 times lower in the smectic-ZA phase than in an ordinary smectic-A liquid crystal. Possible effects of the antiferroelectric layer polarization on the director splay elasticity and viscosity are described. The temperature dependencies of the splay, twist, and bend elastic constants and associated viscosities in the higher temperature nematic phase are also presented.

cond-mat.soft↗