Figure 4.

(A) The map variables used to describe the beat-to-beat evolution of the $APD$$\left(A_n\right)$, the DI $\left(D_n\right)$, the SR load $x_{ij}\left(n\right)$, and the diastolic Ca $c_{ij}\left(n\right)$. The peak of the internal Ca concentration is denoted as $c_{ij}^p\left(n\right)$. (B) The average order parameter $⟨\left|s\right|⟩$ as a function of pacing rate. (C) The susceptibility $\chi$ computed for the indicated system size $L$. (D) A plot of the Binder cumulant $U$. Intersection point is indicative of a phase transition at $T_c$. All averages are computed using ${10}^8$ beats with a standard error that is smaller than the symbol size.