In the frames of field theoretical fixed dimension renormalization group scheme critical exponents of m vector model are calculated in the case when space dimension continuously changes from the lower critical dimension to the upper one.
An exact renormalization equation is derived by making an infinitesimal change in the cutoff in momentum space. From this equation the expansion for critical exponents around dimensionality 4 and the limit $n=\ensuremath{\infty}$ of the n-vector model are calculated. We obtain agreement with the results of Wilson and Fisher, and with the spherical model.
The equivalence of the critical behavior of the d-dimensional Ising model at a finite temperature to that of a d- 1 dimensional system at zero temperature as a function of a transverse magnetic field is used to calculate the critical exponents for the three-dimensional model by applying the Niemeijer-Van Leeuwen renormalization-group method to the two-dimensional problem.
We investigate the critical properties of d-dimensional magnetic systems with quenched extended defects, correlated in ${\ensuremath{\varepsilon}}_{d}$ dimensions (which can be considered as the dimensionality of the defects) and randomly distributed in the remaining $d\ensuremath{-}{\ensuremath{\varepsilon}}_{d}$ dimensions; both in the case of fixed dimension d=3 and when the space dimension continuously changes from the lower critical dimension to the upper one. Our renormalization-group calculations are performed in the minimal subtraction scheme. We analyze the two-loop renormalization-group functions for different fixed values of the parameters $d,{\ensuremath{\varepsilon}}_{d}.$ To this end, we apply the Chisholm-Borel resummation technique and report the numerical values of the critical exponents for the universality class of this system.
This is the third article in a sequence in which we reexamine the calculation of the critical exponents of the N-vector model from field theory. In the first article we have applied summation methods to the ε-expansion for which new terms have been calculated (up to order ε5). In the second article we have incorporated in the summation method, in the case of Ising-like systems (N = 1), the knowledge of the exact values of exponents in d = 2 dimensions. Since exact values are now also known in the case of polymers (N = 0), we apply here the same method to this case. Moreover, since for the exponent η for N = 1 the results coming from the ε-expansion and perturbation series at fixed dimension are slightly different, we have recalculated the exponents in the latter case using the variant of the summation method developed for the ε-expansion. The results do not change significantly showing both the reliability and the limits of the method.