Abstract
Forn spins 1/2 coupled linearly to a boson field in a volumeV n, the existence of the specific free energy is proved in the limitn → ∞,V n → ∞ withn/V n=const. The interaction is essentially of the mean field type, in as much as it is proportional to 1/√V n; the coupling constants are allowed to be spin dependent. A variational expression is obtained for the limiting specific free energy, and a critical temperature is identified above which the system behaves as if there were no coupling at all.
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Raggio, G.A. The free energy of the spin-boson model. J Stat Phys 53, 565–581 (1988). https://doi.org/10.1007/BF01014214
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DOI: https://doi.org/10.1007/BF01014214