Trigonal-to-Tetragonal Transition in Stressed SrTiO3: A Realization of the Three-State Potts Model

The trigonal-to-pseudotetragonal structural first-order transition in [111]-stressed SrTi${\mathrm{O}}_3$ is shown to be a realization of the continuous three-state Potts model. The measured order-parameter discontinuity at the transition, $〈\Delta \overrightarrow{\varphi }〉$, depends on the trigonal order parameter $〈{\varphi }_{\mathrm{}\left[111\right]\mathrm{}}〉\propto M$ as $\left|〈\Delta \overrightarrow{\varphi }〉\right|\propto {\mathrm{}\left|M\right|\mathrm{}}^{{\delta }^{*}}$, with ${\delta }^{*}=0.62\mathrm{}\pm 0.10$ (mean-field theory predicts ${\delta }^{*}=1\mathrm{}$). This agrees with renormalization-group predictions, and proves that the model has a first-order transition even in the fluctuation-dominated region.