Nematic phase in the J1-J2 square-lattice Ising model in an external field

• Autor(a) principal: Guerrero Duymovic, Alejandra Isabel
• Data de Publicação: 2015
• Outros Autores: Stariolo, Daniel Adrian, Almarza, Noé G.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Institucional da UFRGS
• Texto Completo: http://hdl.handle.net/10183/118230
• topic: Método de Monte Carlo; Modelo de ising; Diagramas de fase

The J1-J2 Ising model in the square lattice in the presence of an external field is studied by two approaches: the cluster variation method (CVM) and Monte Carlo simulations. The use of the CVM in the square approximation leads to the presence of a new equilibrium phase, not previously reported for this model: an Ising-nematic phase, which shows orientational order but not positional order, between the known stripes and disordered phases. Suitable order parameters are defined, and the phase diagram of the model is obtained. Monte Carlo simulations are in qualitative agreement with the CVMresults, giving support to the presence of the new Ising-nematic phase. Phase diagrams in the temperature-external field plane are obtained for selected values of the parameter κ = J2/|J1| which measures the relative strength of the competing interactions. From the CVM in the square approximation we obtain a line of second order transitions between the disordered and nematic phases, while the nematic-stripes phase transitions are found to be of first order. The Monte Carlo results suggest a line of second order nematicdisordered phase transitions in agreement with the CVM results. Regarding the stripes-nematic transitions, the presentMonte Carlo results are not precise enough to reach definite conclusions about the nature of the transitions.