Boundary criticality of the O(N) model in d = 3 critically revisited

It is known that the classical O (N) O (N) model in dimension d > 3 d gt; 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extra-ordinary and the special. For the ordinary transition the bulk and the boundary order simultaneously; the extra-ordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3 d = 3, it is less clear what happens to the extra-ordinary and special fixed points when d = 3 d = 3 and N 2 N ≥ 2. Here we show that formally treating N N as a continuous parameter, there exists a critical value Nc > 2 N c gt; 2 separating two distinct regimes. For 2 N 2 ≤ N N c the extra-ordinary fixed point survives in d = 3 d = 3, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of r log r. For N > Nc N gt; N c there is no fixed point with order parameter correlations decaying slower than power law. We discuss several scenarios for the evolution of the phase diagram past N = Nc N = N c. Our findings appear to be consistent with recent Monte Carlo studies of classical models with N = 2 N = 2 and N = 3 N =