Effective Field Methods for Magnetic Systems
G. Kamieniarz, R. Dekeyser
Institute of Theoretical Physics, KU Leuven, Belgium

and G. Musiał
Institute of Physics, A. Mickiewicz University, Matejki 48/49, 60-769 Poznań, Poland
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The self-consistency conditions inherent in the mean-field approximation are extended to take into account some pair correlations. Ii is shown that this extension is systematically improvable and applicable to the spin systems with short-range interactions in the entire temperature region. With two correlations included and the renormalization-group ideas implemented, the critical properties of the Ising model are satisfactorily revealed. Moreover, the improved estimates of the thermodynamical quantities for the quantum linear XY model from the finite-size calculations are found down to very low temperatures.
DOI: 10.12693/APhysPolA.85.413
PACS numbers: 64.60.Cn, 64.60.Fr, 05.50.+q, 75.10.Dg, 75.10.Hk, 75.10.Jm, 75.40.Cx, 75.40.Mg