Perturbative expansion

Loop expansion around the Bethe solution for the random magnetic field Ising ferromagnets at zero temperature

We apply to the random-field Ising model at zero temperature (T = 0) the perturbative loop expansion around the Bethe solution. A comparison with the standard ε expansion is made, highlighting the key differences that make the expansion around the Bethe solution much more appropriate to correctly describe strongly disordered systems, especially those controlled by a T = 0 renormalization group (RG) fixed point. The latter loop expansion produces an effective theory with cubic vertices.

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