Study of the Landau bicritical point in dense systems of hard biaxial molecules

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Abstract
We study two systems of hard biaxial molecules: hard spheroplatelets and hard ellipsoids, using the liquid-crystalline version of the smoothed density approximation (SDA). The first system is studied for all elongations of the spheroplatelet c, whereas the second only for c ⩽ 7. For both systems, we locate the line of Landau bicritical points at which a direct transition from the isotropic phase to the biaxial phase occurs. We find that the density of the isotropic phase at the Landau bicritical point is always higher than that at the isotropic-nematic transition in the limit of uniaxial molecules and the difference ranges from 10 to 30 per cent. For hard ellipsoids, we obtain a similar scaling behaviour at the Landau bicritical point as for spheroplatelets, i.e. b ∼c 1/2, where b denotes the breadth of the ellipsoid.