Annealed scaling for a charged polymer

Abstract

We study an undirected polymer chain living on the 1-dimensional integer lattice and carrying i.i.d.\ random charges. Each self-intersection of the polymer contributes to the Hamiltonian an energy that is equal to the product of the charges of the two monomers that meet. The joint law for the polymer chain and the charges is given by the Gibbs distribution associated with the Hamiltonian. The focus is on the \emph{annealed free energy} per monomer in the limit as polymer length diverges. We derive a \emph{spectral representation} for the free energy and we exhibit a critical curve in the parameter plane of charge bias versus inverse temperature separating a \emph{ballistic phase} from a \emph{subballistic phase}. The phase transition is \emph{first order}. We prove large deviation principles for the laws of the empirical speed and the empirical charge, and derive a spectral representation for the associated rate functions. Interestingly, in both phases both rate functions exhibit \emph{flat pieces}, which correspond to an inhomogeneous strategy for the polymer to realise a large deviation. The large deviation principles lead to laws of large numbers and central limit theorems. We identify the scaling behavior of the critical curve for small and large charge bias and also the scaling behavior of the free energy for small charge bias and small inverse temperature. Both are linked to an associated \emph{Sturm-Liouville eigenvalue problem}. A key tool in our analysis is the Ray-Knight formula for the local times of 1-dimensional simple random walk. It is used to derive a \emph{closed form expression} for the generating function of the annealed partition function and to derive the spectral representation for the free energy. What happens for the \emph{quenched free energy} remains open. We state two modest results and raise a few questions.