Calamitic and antinematic orientational order produced by the generalized Straley lattice model

摘要

We consider here a classical model, consisting of D2h-symmetric particles in a three-dimensional simple-cubicnlattice; the pair potential is isotropic in orientation space, and restricted to nearest neighbors. The simplest potentialnmodel is written in terms of the squares of the scalar products between unit vectors describing the three interactingnarms of the molecules, as proposed in previous literature. Two predominant antinematic couplings of equalnstrength (+1) are perturbed by a comparatively weaker calamitic one, parameterized by a coupling constant −znranging in [−1,0]. This choice rules out thermodynamically stable phases endowed with macroscopic biaxiality.nThe antinematic terms favor states with the corresponding molecular axes mutually orthogonal. Although thenlow-temperature phase of the special case with null calamitic term (PP0) is uniaxial and antinematically ordered,nin the general case presented here both Monte Carlo and molecular-field approaches show that, for z close tonzero, the models exhibit a low-temperature uniaxial nematic phase, followed by an antinematic one, and finallynby the orientationally disordered one. On the other hand, for sufficiently large values of z, we only find evidencenof uniaxial calamitic behavior, as expected by following the limiting cases.