For systems that exhibit a second-order phase transition with a spontaneously broken continuous $\mathrm{O}\left(N\right)$ symmetry at low temperatures, we give a criterion for judging at which temperature $T_K$ long-range directional fluctuations of the order field destroy the order when approaching the critical temperature from below. The temperature $T_K$ lies always significantly below the famous Ginzburg temperature $T_G$ at which size fluctuations of finite range become important.

• Received 17 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.286