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The hypothesis of two-scale-factor universality, originally proposed by Stauffer, Ferer, and Wortis, is shown to follow from the renormalization-group approach, for systems close to their critical point. Values of the universal ratios involving correlation length and specific-heat amplitudes are obtained from the expansion, for Ising, X-Y, and Heisenberg models. In the latter two cases the correlation function has a power-law behavior at large distances below T₂, and the (transverse) correlation length is defined in terms of the stiffness constant ₒ. Experimental values of the correlation lengths and amplitude ratios are determined for superfluid ^4He, which is X-Y-like, and for the Heisenberg antiferromagnet RbMnF₃. Comparisons are made between the values of the amplitude ratios coming from expansions, series, and experiments.
Hohenberg et al. (Thu,) studied this question.