Improved harmonic approximation and the 2D Ising model at T ≠ Tc and h ≠ 0

@article{Iucci2002ImprovedHA,
  title={Improved harmonic approximation and the 2D Ising model at T ≠ Tc and h ≠ 0},
  author={An{\'i}bal Iucci and Carlos M. Na'on},
  journal={Journal of Physics A},
  year={2002},
  volume={35},
  pages={8005-8012}
}
We propose a new method to determine the unknown parameter associated with a self-consistent harmonic approximation. We check the validity of our technique in the context of the sine-Gordon model. As a non-trivial application we consider the scaling regime of the 2D Ising model away from the critical point and in the presence of a magnetic field h. We derive an expression that relates the approximate correlation length ξ, T − Tc and h. 

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