Semianalytical solutions of Ising-like and Potts-like magnetic polymers on the Bethe lattice

Abstract

We study magnetic polymers, defined as self-avoiding walks where each monomer $i$ carries a "spin'' $s_i$ and interacts with its first neighbor monomers, let us say $j$, via a coupling constant $J(s_i,s_j)$. Ising-like [$s_i = \pm 1$, with $J(s_i,s_j) = \varepsilon s_i s_j$] and Potts-like [$s_i = 1,\ldots,q$, with $J(s_i,s_j)=\varepsilon_{s_i} \delta(s_i,s_j)$] models are investigated. Some particular cases of these systems have recently been studied in the continuum and on regular lattices, and are related to interesting applications. Here, we solve these models on Bethe lattices of ramification $\sigma$, focusing on the ferromagnetic case in zero external magnetic field. In most cases, the phase diagrams present a non-polymerized (NP) and two polymerized phases: a paramagnetic (PP) and a ferromagnetic (FP) one. However, quite different thermodynamic properties are found depending on $q$ in the Potts-like polymers and on whether one uses the Ising or Potts coupling in the two-state systems. Importantly, these results indicate that when $q\le 6$ the spin ordering transition is preceded by the polymer collapse transition, whereas for $q\ge 7$ and in the Ising case these transitions happen together at critical-end-points. Some interesting non-standard Potts models are also studied, such as the lattice version of the model for epigenetic marks in the chromatin introduced in [PRX {\bf 6}, 041047 (2016)]. In addition, the solution of the dilute Ising and dilute Potts models on the Bethe lattice are also presented here, once they are important to understand the PP-FP transitions.