Fixation for $\mathcal{U}$-Ising and $\mathcal{U}$-voter dynamics with frozen vertices

Abstract

The zero-temperature stochastic Ising model is a special case of the famous stochastic Ising model of statistical mechanics, and the voter model is another classical model in this field. In both models, each vertex of the graph $\mathbb{Z}^d$ can have one of two states, and can change state to match the state of its neighbors. In 2017, Morris proposed generalizations of these models, the $\mathcal{U}$-Ising and $\mathcal{U}$-voter dynamics, in which a vertex can change state to match the state of certain subsets of vertices near it. These generalizations were inspired by similar generalizations in the related model of bootstrap percolation, where Balister, Bollobás, Duminil-Copin, Morris, Przykucki, Smith and Uzzell were able to establish a very impressive universality classification of the generalized models. However, there have been very few results on the $\mathcal{U}$-Ising and $\mathcal{U}$-voter dynamics. The only one is due to Blanquicett in 2021, who obtained a few encouraging advances on the important question of fixation, which is only partially solved for the zero-temperature stochastic Ising model: will all vertices eventually settle on a given state or will they oscillate forever between the two states? In this work, we tackle a question which was solved for the zero-temperature stochastic Ising model by Damron, Eckner, Kogan, Newman and Sidoravicius in 2015: fixation when a fraction of the vertices of $\mathbb{Z}^d$ are frozen in one of the states. For $d=1$ and $2$, in most cases we prove that if all frozen vertices are in the same state, all vertices eventually settle at this state. Moreover, if vertices can be frozen in both states but the proportion of vertices frozen in the second state is small enough, we were able to establish a universality classification identifying the models in which all vertices settle in a given state.