On the uniqueness of Gibbs measure in the Potts model on a Cayley tree with external field

Abstract: The paper concerns the $q$-state Potts model (i.e., with spin values in ${1,\dots,q}$) on a Cayley tree $\mathbb{T}^k$ of degree $k\geq 2$ (i.e., with $k+1$ edges emanating from each vertex) in an external (possibly random) field. We construct the so-called splitting Gibbs measures (SGM) using generalized boundary conditions on a sequence of expanding balls, subject to a suitable compatibility criterion. Hence, the problem of existence/uniqueness of SGM is reduced to solvability of the corresponding functional equation on the tree. In particular, we introduce the notion of translation-invariant SGMs and prove a novel criterion of translation invariance. Assuming a ferromagnetic nearest-neighbour spin-spin interaction, we obtain various sufficient conditions for uniqueness. For a model with constant external field, we provide in-depth analysis of uniqueness vs.\ non-uniqueness in the subclass of completely homogeneous SGMs by identifying the phase diagrams on the "temperature--field" plane for different values of the parameters $q$ and $k$. In a few particular cases (e.g., $q=2$ or $k=2$), the maximal number of completely homogeneous SGMs in this model is shown to be $2^q-1$, and we make a conjecture (supported by computer calculations) that this bound is valid for all $q\ge 2$ and $k\ge2$.