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Phase diagram of the Ising antiferromagnet with nearest-neighbor and next-nearest-neighbor interactions on a square lattice
Authors:Rosana A dos Anjos  J Ricardo de Sousa
Institution:a Departamento de Física, Universidade Federal do Amazonas, 3000 Japiim, 69077-000 Manaus-AM, Brazil
b Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, CP 702, 30123-970 Belo Horizonte-MG, Brazil
Abstract:The phase diagram of the Ising model in the presence of nearest-neighbor (J1) and next-nearest-neighbor (J2) interactions on a square lattice is studied within the framework of the differential operator technique. The Hamiltonian is solved by effective-field theory in finite cluster (we have chosen N=4 spins). We have proposed a functional for the free energy (similar to Landau expansion) to obtain the phase diagram in the (T,α) space (α=J2/J1), where the transition line from the superantiferromagnetic (SAF) to the paramagnetic (P) phase is of first-order in the range 1/2<α<0.95 in contrast to previous study of CVM (Cluster Variational Method) that predict first-order transition for α=1.0. Our results for α=1.0 are in accordance with MC (Monte Carlo) simulations, that predict a second-order transition.
Keywords:05  50  +q  64  60  Cn  75  10  Hk  75  40  Cx
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