Feynman-Graph Expansion for Critical Exponents

The critical exponents γ, η and the "crossover index" φ are computed for generalized classical Heisenberg models with n internal degrees of freedom as an exact expansion in ε=4-d (d is the number of space dimensions). Results are obtained to order ε² for γ and to order ε³ for η. The results to this order for the three-dimensional Ising case (n=ε=1) are γ=1.244 and η=0.037.