Effect of an external magnetic field on the nematic-isotropic phase transition in mesogenic systems of uniaxial and biaxial molecules

TL;DR

This study uses Monte Carlo simulations to analyze how external magnetic fields influence the nematic-isotropic phase transition in uniaxial and biaxial liquid crystal molecules, revealing a more rapid transition temperature shift than traditional theories.

Contribution

It provides a detailed computational analysis of magnetic field effects on nematic-isotropic transitions, highlighting differences between uniaxial and biaxial molecules and challenging existing theoretical predictions.

Findings

Magnetic field causes a more rapid shift in transition temperature than Landau-de Gennes and Maier-Saupe theories.

Biaxial molecules exhibit a higher transition temperature shift compared to uniaxial molecules under the same magnetic field.

The external magnetic field weakens the nematic-isotropic transition, especially for biaxial molecules, with a critical field estimated at around 110 T.

Abstract

Influence of an external magnetic field on the nematic-isotropic ($N-I$) phase transition in a dispersion model of nematic liquid crystals, where the molecules are either perfectly uniaxial or biaxial (board-like), has been studied by Monte Carlo simulation. Using multiple histogram reweighting technique and finite size scaling analysis the order of the phase transition, the transition temperature at the thermodynamic limit and the stability limit of the isotropic phase below the transition temperature for different magnetic field strengths have been determined. The magnetic field dependence of the shift in $N-I$ transition temperature is observed to be more rapid than that predicted by the standard Landau-de Gennes and Maier-Saupe mean field theories. We have shown that for a given field strength the shift in the transition temperature is higher for the biaxial molecules in comparison with the uniaxial case. The study shows that the $N-I$ transition for the biaxial molecules is weaker than the well known weak first order $N-I$ transition for the uniaxial molecules and the presence of the external magnetic field (up to a certain critical value) makes the transition much more weaker for both the systems. The estimate of the critical magnetic field ($\sim 110 T$) for the common nematics is found to be smaller than the earlier estimates.