Evidence for algebraic orientational order in a two-dimensional hard-core nematic

We present Monte Carlo simulations on a two-dimensional (2D) fluid of N infinitely thin hard rods of length L (N3200). This system has an isotropic phase at low densities and a ``nematic'' phase at high densities. Although true long-range orientational order is not excluded a priori, the simulations indicate that the nematic phase has algebraic order. We find no evidence for a first-order isotropic-nematic transition; rather, all the available evidence points towards the occurrence of a disclination-unbinding transition of the Kosterlitz-Thouless type. The heat capacity C₏ peaks at a density some 20% below the estimated disclination-unbinding transition point. We discuss earlier simulations on 2D nematics in the light of the current results. We have computed the virial coefficients of the hard-needle fluid up to B₅ and find that B₄ is very small, while B₅ is negative.