Grand canonical and canonical solution of self-avoiding walks with up to three monomers per site on the Bethe lattice

TL;DR

This paper analytically solves a polymer model with up to three monomers per site on the Bethe lattice, revealing a complex phase diagram with continuous transitions, and compares theoretical predictions with existing simulations.

Contribution

It provides an exact grand-canonical solution for a self-avoiding walk model with multiple visits per site on the Bethe lattice, exploring its phase behavior and ensemble relationships.

Findings

Rich phase diagram with coexistence and critical surfaces

Transition between phases is always continuous in the model

Comparison with simulations shows some discrepancies in transition nature

Abstract

We solve a model of polymers represented by self-avoiding walks on a lattice which may visit the same site up to three times in the grand-canonical formalism on the Bethe lattice. This may be a model for the collapse transition of polymers where only interactions between monomers at the same site are considered. The phase diagram of the model is very rich, displaying coexistence and critical surfaces, critical, critical endpoint and tricritical lines, as well as a multicritical point. From the grand-canonical results, we present an argument to obtain the properties of the model in the canonical ensemble, and compare our results with simulations in the literature. We do actually find extended and collapsed phases, but the transition between them, composed by a line of critical endpoints and a line of tricritical points, separated by the multicritical point, is always continuous. This result is at variance with the simulations for the model, which suggest that part of the line should be a discontinuous transition. Finally, we discuss the connection of the present model with the standard model for the collapse of polymers (self-avoiding self-attracting walks), where the transition between the extended and collapsed phases is a tricritical point.