XY-to-Gaussian crossover for the Potts-model transition near the bicritical point of stressed SrTiO3

The trigonal-to-pseudotetragonal structural first-order transition in [111]-stressed SrTi${\mathrm{O}}_3$, realizing the three-state Potts model, is studied in the close vicinity of the bicritical point (BCP). The exponents describing the effects of fluctuations on this transition are shown to cross over from $\mathrm{XY}$- to Gaussian-model values as the BCP is approached. The first-order line is shown to approach the BCP tangentially to the temperature axis.