03-107 Vieri Mastropietro

Ising models with four spin interaction at criticality (141K, TeX) Mar 12, 03

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Abstract. We consider two bidimensional Ising models coupled by an interaction quartic in the spins. The model contains both the Eight vertex and the Ashkin Teller models for suitable values of the parameters. By Renormalization Group methods we write a convergent perturbative expansion for the specific heat and for the energy-energy correlation up to the critical temperature. A form of nonuniversality is proved, in the sense that the critical behaviour is described in terms of critical indices which are non trivial functions of the coupling. The logarithmic singularity of the specific heat of the Ising model is removed or changed in a power law (with a non universal critical index) depending on the sign of the interaction.

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