Wide crossover in the 3D Ashkin-Teller model

The phase transition line, for which the possibility of the non-universal behavior studied by Monte Carlo (MC) simulations had been announced, in the standard 3D Ashkin-Teller (AT) model phase diagram is carefully investigated. The results of our MC experiments for transitions between paramagnetic, mixed, and Baxter phases for negative coupling between neighboring spin pairs, the most complex and least recognized region in the phase diagram, are presented. Binder, Challa, and also for the first time of Lee-Kosterlitz cumulants, the last two adapted to the AT model, are exploited. The uncommon coexistence of continuous and first order phase transitions along the same phase boundary on different components of the order parameter are reported. The paper is closed with the conclusion of a wide crossover effect that covers the whole line between tricritical points, where earlier papers indicate the possibility of non-universal behavior with the critical exponent values varying continuously.