Nonuniversal power laws and crossover from critical to classical behavior

Using accurate calculations of the critical-to-classical crossover performed within the ${\mathrm{\phi }}^4$ theory, we show that it is possible to observe nonuniversal power laws extending over almost three decades of temperature as seems to be the case near the cloud point of micellar solutions. The possibility of a nonmonotonic crossover is quantitatively examined and confronted with measurements made near the critical point of xenon. We emphasize the difficulties of determining the mechanism responsible for the nonmonotonicity.