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Spontaneous onset of a low temperature topologically ordered phase in a 2-dimensional (2D) lattice model of uniaxial liquid crystal (LC) was debated extensively pointing to a suspected underlying mechanism affecting the RG flow near the topological fixed point. A recent MC study clarified that a prior crossover leads to a transition to nematic phase. The crossover was interpreted as due to the onset of a perturbing relevant scaling field originating from the extra spin degree of freedom. As a counter example and in support of this hypothesis, we now consider V-shaped bent-core molecules with rigid rod-like segments connected at an assigned angle. The two segments of the molecule interact with the segments of all the nearest neighbours on a square lattice, prescribed by a biquadratic interaction. We compute equilibrium averages of different observables with Monte Carlo techniques as a function of temperature and sample size. For the chosen molecular bend angle and symmetric inter-segment interaction between neighbouirng molecules, the 2D system shows two transitions as a function of T: the higher one at T1 leads to a topological ordering of defects associated with the major molecular axis without a crossover, imparting uniaxial symmetry to the medium described by the first fundamental group of the order parameter space $\pi_{1}$= $Z_{2}$ (inversion symmetry). The second at T2 leads to a medium displaying biaxial symmetry with $\pi_{1}$ = Q (quaternion group). The biaxial phase shows a self-similar microscopic structure with the three axes showing power law correlations with vanishing exponents as the temperature decreases., Comment: 10 pages, 11 figures

The presence of stable topological defects in a two-dimensional (\textit{d} = 2) liquid crystal model allowing molecular reorientations in three dimensions (\textit{n} = 3) was largely believed to induce defect-mediated Berzenskii-Kosterlitz-Thouless (BKT) type transition to a low temperature phase with quasi long-range order. However, earlier Monte Carlo (MC) simulations could not establish certain essential signatures of the transition, suggesting further investigations. We study this model by computing its equilibrium properties through MC simulations, based on the determination of the density of states of the system. Our results show that, on cooling, the high temperature disordered phase deviates from its initial progression towards the topological transition, crossing over to a new fixed point, condensing into a nematic phase with exponential correlations of its director fluctuations. The thermally induced topological kinetic processes continue, however limited to the length scales set by the nematic director fluctuations, and lead to a second topological transition at a lower temperature. We argue that in the (\textit{d} = 2, \textit{n} = 3) system with a biquadratic Hamiltonian, the presence of additional molecular degree of freedom and local $Z_{2}$ symmetry associated with lattice sites, together promote the onset of an additional relevant scaling field at matching length scales in the high temperature region, leading to a crossover., Comment: 5 pages, 7 figures

General quadratic Hamiltonian models, describing interaction between crystal molecules (typically with $D_{2h}$ symmetry) take into account couplings between their uniaxial and biaxial tensors. While the attractive contributions arising from interactions between similar tensors of the participating molecules provide for eventual condensation of the respective orders at suitably low temperatures, the role of cross-coupling between unlike tensors is not fully appreciated. Our recent study with an advanced Monte Carlo technique (entropic sampling) showed clearly the increasing relevance of this cross term in determining the phase diagram, contravening in some regions of model parameter space, the predictions of mean field theory and standard Monte Carlo simulation results. In this context, we investigated the phase diagrams and the nature of the phases therein, on two trajectories in the parameter space: one is a line in the interior region of biaxial stability believed to be representative of the real systems, and the second is the extensively investigated parabolic path resulting from the London dispersion approximation. In both the cases, we find the destabilizing effect of increased cross-coupling interactions, which invariably result in the formation of local biaxial organizations inhomogeneously distributed. This manifests as a small, but unmistakable, contribution of biaxial order in the uniaxial phase.The free energy profiles computed in the present study as a function of the two dominant order parameters indicate complex landscapes, reflecting the difficulties in the ready realization of the biaxial phase in the laboratory., Comment: 23 pages, 12 figures

Phase sequences of the biaxial nematic liquid crystal in the interior of the essential triangle are studied with Wang Landau sampling. The evidence points to the existence of an intermediate unixial phase with low biaxiality in the isotropic to biaxial nematic phase sequence., Comment: 25 pages, 16 figures. arXiv admin note: text overlap with arXiv:1509.03834

Investigations of the phase diagram of biaxial liquid crystal systems through analyses of general Hamiltonian models within the simplifications of mean-field theory (MFT), as well as by computer simulations based on microscopic models, are directed towards an appreciation of the role of the underlying molecular-level interactions to facilitate its spontaneous condensation into a nematic phase with biaxial symmetry. Continuing experimental challenges in realising such a system unambiguously, despite encouraging predictions from MFT for example, are requiring more versatile simulational methodologies capable of providing insights into possible hindering barriers within the system, typically gleaned through its free energy dependences on relevant observables as the system is driven through the transitions. The recent brief report from this group [B. Kamala Latha, et. al., Phys. Rev. E 89, 050501 (R), 2014] summarizing the outcome of detailed Monte Carlo simulations carried out employing entropic sampling technique, suggested a qualitative modification of the MFT phase diagram as the Hamiltonian is asymptotically driven towards the so-called partly-repulsive regions. It was argued that the degree of the (cross) coupling between the uniaxial and biaxial tensor components of neighbouring molecules plays a crucial role in facilitating, or otherwise, a ready condensation of the biaxial phase, suggesting that this could be a plausible f actor in explaining the experimental difficulties. In this paper, we elaborate this point further, providing additional evidences from curious variations of free-energy profiles with respect to the relevant orientational order parameters, at different temperatures bracketing the phase transitions., Comment: 31 pages, 13 figures

Equilibrium director structures in two thin hybrid planar films of biaxial nematics are investigated through Markov chain Monte Carlo simulations based on a lattice Hamiltonian model within the London dispersion approximation. While the substrates of the two films induce similar anchoring influences on the long axes of the liquid crystal molecules (viz. planar orientation at one end and perpendicular, or homeotropic, orientations at the other), they differ in their coupling with the minor axes of the molecules. In Type-A film the substrates do not interact with the minor axes at all (which is experimentally relatively more amenable), while in Type-B, the orientations of the molecular axes at the surface layer are influenced as well by their biaxial coupling with the surface. Both films exhibit expected bending of the director associated with ordering of the molecular long axes due to surface anchoring. Simulation results indicate that the Type-A film hosts stable and noise free director structures in the biaxial nematic phase of the LC medium, resulting from dominant ordering of one of the minor axes in the plane of the substrates. High degree of this stable order thus developed could be of practical interest for in-plane switching applications with an external field. Type-B film, on the other hand, experiences competing interactions among the minor axes, due to incompatible anchoring influences at the bounding substrates, apparently leading to frustration, and hence to noisy equilibrium director structures., Comment: 22 pages, 13 figures