Monte Carlo simulation of two-dimensional hard ellipses

We report constant-pressure Monte Carlo simulations of a system of hard ellipses with aspect ratios k=2, 4, and 6. The results show three different phases: isotropic, nematic, and solid, except for k=2, where no nematic phase is formed. In all cases the fluid-solid transition appears to be first order. However, the isotropic-nematic transition looks first order for k=4 and continuous, via disclination unbinding, for k=6. Thus a tricritical point for some k between these two values is predicted. The simulation results are also compared with a previous theoretical calculation. Satisfactory agreement is found for the equation of state, but the theoretical prediction for the location of the isotropic-nematic transition disagrees with the simulation results. Finally, we observed that the solid phase near melting exhibits anomalously large fluctuations.