University in Analytic Corrections to Scaling for Planar Ising Models

It is argued that the leading corrections to scaling for planar Ising models, which occur as analytic factors, arise from the quadratic terms of the nonlinear thermal and ordering fields (rather than from irrelevant variables). This yields αeff^'+2βeff+γeff^'=2 for the effective critical exponents, with no leading corrections, and is confirmed by exact square-lattice results for arbitrary anisotropy J₂J₁. For isotropic lattices the ratios of correction amplitudes and quadratic nonlinear-field terms appear universal.