Critical exponents by the scaling-field method: The isotropicN-vector model in three dimensions

A numerical technique, termed the scaling-field method, is developed for solving by successive approximation Wilson's exact renormalization-group equation for critical phenomena in three-dimensional spin systems. The approach uses the scaling-field representation of the Wilson equation derived by Riedel, Golner, and Newman. A procedure is proposed for generating in a nonperturbative and unbiased fashion sequences of successively larger truncations to the infinite hierarchy of scaling-field equations. A "principle of balance" is introduced and used to provide a self-consistency criterion. The approach is then applied to the isotropic $N$-vector model. Truncations to order 13 (10, when $N=1\mathrm{}$) scaling-field equations yield the leading critical exponents, $\nu$ and $\eta$, and several of the correction-to-scaling exponents, ${\Delta }_m$, to high precision. Results for $N=0, 1, 2, \mathrm{and} 3$ are tabulated. For the Ising case ($N=1\mathrm{}$), the estimates $\nu =0.626\mathrm{}\pm 0.009$, $\eta =0.040\mathrm{}\pm 0.007$, and ${\Delta }_1\equiv {\Delta }_{400}=0.54\mathrm{}\pm 0.05$ are in good agreement with recent high-temperature-series results, though exhibiting larger confidence limits at the present level of approximation. For the first time, estimates are obtained for the second and third correction-to-scaling exponents. For example, for the Ising model the second "even" and first "odd" correction-to-scaling exponents are ${\Delta }_{422}=1.67\mathrm{}\pm 0.11$ and ${\Delta }_{500}=1.5\mathrm{}\pm 0.3$, respectively. Extensions necessary to improve the accuracy of the calculation are discussed, while applications of the approach to anisotropic $N$-vector models are described elsewhere. Finally, the scaling-field method is compared with other techniques for the high-precision calculation of critical phenomena in three dimensions, i.e., high-temperature-series, Monte Carlo renormalization-group, and field-theoretic perturbation expansions.