Soumen Kumar Roy

Splay and bend elastic constants and rotational viscosity coefficient in a mixture of 4-4′-n-pentyl-cyanobiphenyl and 4-4′-n-decyl-cyanobiphenyl

Abstract Measurement of splay, bend elastic constants and rotational viscosity coefficient in mixtures of 4-4′- n -pentyl-cyanobiphenyl (5CB) and 4-4′- n -decyl-cyanobiphenyl (10CB) are reported. The mixture exhibits smectic A and nematic phases. The bend elastic constant and the rotational viscosity coefficient diverge as the smectic A phase is approached from the nematic side and we have determined the exponents for divergence of these two quantities. Both exponents seem to depend on the relative concentration of the constituents in the mixtures, and the values for the bend elastic constant show a jump from 0.7 to 1.0 when the mole fraction of 5CB increases from 0.25 to 0.30. This is consistent with the presence of a tricritical point for the phase transition. The possibility of the existence of a smecticA⇌nematic tricritical point is discussed in the framework of Landau theory.

Finite size scaling in the planar Lebwohl–Lasher model

Abstract The standard finite size scaling method for second order phase transition has been applied to Monte Carlo data obtained for a planar Lebwohl–Lasher lattice model using the Wolff cluster algorithm. We obtain T c and the exponents γ , ν , and z and the results are different from those obtained by other investigators.

Performance of Wang–Landau algorithm in continuous spin models and a case study: Modified XY-model

Abstract Performance of Wang–Landau (WL) algorithm in two continuous spin models is tested by determining the fluctuations in energy histogram. Finite size scaling is performed on a modified XY-model using different WL sampling schemes. Difficulties faced in simulating relatively large continuous systems using WL algorithm are discussed.

Computer simulation of two continuous spin models using Wang–Landau-Transition-Matrix Monte Carlo algorithm

Abstract Monte Carlo simulation using a combination of Wang–Landau (WL) and Transition Matrix (TM) Monte Carlo algorithms to simulate two lattice spin models with continuous energy is described. One of the models, the one-dimensional Lebwohl–Lasher model has an exact solution and we have used this to test the performance of the mixed algorithm (WLTM). The other system we have worked on is the two-dimensional XY-model. The purpose of the present work is to test the performance of the WLTM algorithm in continuous models and to suggest methods for obtaining best results in such systems using this algorithm.

Monte Carlo simulation of joint density of states in one-dimensional Lebwohl–Lasher model using Wang–Landau algorithm

Abstract Monte Carlo simulation using the Wang–Landau algorithm has been performed in an one-dimensional Lebwohl–Lasher model. Both one-dimensional and two-dimensional random walks have been carried out. The results are compared with the exact results which are available for this model.

Dynamical scaling in two-dimensional quenched uniaxial nematic liquid crystals

The phase-ordering kinetics of the two-dimensional uniaxial nematic has been studied using a cell dynamic scheme. The system after quench from T=infinity was found to scale dynamically with an asymptotic growth law similar to that of the two-dimensional O(2) model (quenched from above the Kosterlitz-Thouless transition temperature), i.e., L (t) approximately [t/ln (t/ t(0) ) ](1/2) (with nonuniversal time scale t(0) ). We obtained the true asymptotic limit of the growth law by performing our simulation for a sufficiently long time. The presence of topologically stable 1/2 -disclination points is reflected in the observed large-momentum dependence k(-4) of the structure factor. The correlation function was also found to tally with the theoretical prediction of the correlation function for the two-dimensional O(2) system.

Effect of cadmium sulfide nanorod content on Freedericksz threshold voltage, splay and bend elastic constants in liquid-crystal nanocomposites

This report describes how doping liquid crystals (LC) with rod-like hexagonal semiconductor nanoprisms alters the dielectric and elastic properties of the composites as compared with a pristine nematic liquid crystal (NLC). Cadmium sulfide nanorods were synthesized via the solvothermal process and blended with a NLC. Nanorods were highly miscible with NLC and produced a topological defect-free texture up to a certain limit. A good dark state was achieved during the homeotropic configuration of the cell within that limit. Appreciable changes in splay and bend elastic constants of the LCs were observed after blending with nanorods. Long-range order was established in the hybrid system, and consequently the anisotropy was increased. The threshold voltage decreased dramatically by ?31%. Dielectric study revealed a high-frequency mode, which might be due to anchoring of the LC with nanorods.

Phase transitions in two planar lattice models and topological defects: a Monte Carlo study.

Monte Carlo simulation has been performed in the planar P2 and P4 models to investigate the effects of the suppression of topological defects on the phase transition exhibited by these models. Suppression of the 1/2 defects on the square plaquettes in the P2 model leads to complete elimination of the phase transition observed in this model. However, in the P4 model, on suppressing the single 1/2 defects on square plaquettes, the otherwise first order phase transition changes to a second order one which occurs at a higher temperature, and this is due to the presence of a large number of 1/2 pair defects which are left within the square plaquettes. When we suppressed these charges too, complete elimination of the phase transition was observed.

Effect of a rigid, non-polar solute on the dielectric anisotropy, the splay and bend elastic constants, and on the rotational viscosity coefficient of 4-4'-n-heptylcyanobiphenyl

The effect of mixing a rigid, non-polar, non-mesogenic solute, biphenyl (C6H5-C6H5), in the nematic solvent 7CB (4,4′-n-heptylcyanobiphenyl) is investigated. The solute is found to reduce the nematic order and a two-phase region appears. We report measurements of the transition temperatures, dielectric anisotropy, and splay and bend elastic constants, as well as the rotational viscosity coefficient by the method of electric field-induced Fréedericksz transition for biphenyl concentrations up to 8.0%.

Importance of transverse dipoles in the stability of biaxial nematic phase: A Monte Carlo study

Monte Carlo simulation performed on a lattice system of biaxial molecules possessing D 2h symmetry and interacting with a second rank anisotropic dispersion potential yields three distinct macroscopic phases depending on the biaxiality of the constituent molecules. The phase diagram of such a system as a function of molecular biaxiality is greatly modified when a transverse dipole is considered to be associated with each molecule so that the symmetry is reduced to C 2v . Our results indicate the splitting of the Landau point, i.e. the point in the phase diagram where a direct transition from the isotropic phase to the biaxial nematic phase occurs, into a Landau line for a system of biaxial molecules with strong transverse dipoles. The width of the Landau line becomes maximum for an optimal value of the relative dipolar strength. The presence of transverse dipoles leads to the stabilization of the thermotropic biaxial nematic phase at higher temperature and for a range of values of molecular biaxiality. The structural properties in the uniaxial and biaxial phases are investigated by evaluating the first rank and second rank orientational correlation functions. The dipole-induced long-range order of the anti-ferroelectric structure in the biaxial nematic phase, is revealed.