Journal of Physical Studies 4(4), 454–463 (2000) DOI: https://doi.org/10.30970/jps.04.454 THERMODYNAMICS OF THE TRANSVERSE FIELD ISING MODEL WITHIN CLUSTER APPROXIMATION R. R. Levitskii, O. R. Baran Institute for Condensed Matter Physics of the National Academy of Science of Ukraine 1 Svientsitskii Str., Lviv, UA–79011, Ukraine

The transverse field Ising model was investigated within two-particle cluster approximation with two and one variational parameters. The long-range interaction was taken into account in the framework of the mean-field approximation.

At various values of transverse field and long-range interaction, phase diagrams were constructed and temperature dependences of the order parameter and longitudinal static susceptibility were calculated. It was shown that cluster approximation with two variational parameters gave qualitatively incorrect results in a low-temperature region and that cluster approximation with one variational parameter is not suitable for one-dimensional lattices.

PACS number(s): 75.10.Jm, 77.80.Bh

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