The 2×1 and 2×2 Structures on a Triangular Lattice

抄録

Phase transitions of the triangular Ising model in a field with nearest and second nearest neighbor antiferromagnetic interactions with couplings, −J and −J⁄2, respectively, are studied by the interfacial free energy method, by the mean field approximation, by the transfer matrix and phenomenological renormalization methods and by the Monte Carlo simulation. At low temperatures and at fields weaker than the lower critical field h_c1 the system takes the 2×1 structure, and at fields between h_c1 and the upper critical field h_c2 it takes the 2×2 structure. Between the 2×1 and 2×2 phases near the field h_c1, a partial disordered phase, in which a part of the sublattices are in disorder, is found. Specific heat shows no divergence, when the partial disordered phase changes to other phases.