Copolymer at selective interfaces and pinning potentials: weak coupling limits

Abstract

We consider a simple random walk of length $N$, denoted by $(S_{i})_{i\in \{1,...,N\}}$, and we define $(w_i)_{i\geq 1}$ a sequence of centered i.i.d. random variables. For $K\in\N$ we define $((\gamma_i^{-K},...,\gamma_i^K))_{i\geq 1}$ an i.i.d sequence of random vectors. We set $\beta\in \mathbb{R}$, $\lambda\geq 0$ and $h\geq 0$, and transform the measure on the set of random walk trajectories with the Hamiltonian $\lambda \sum_{i=1}^{N} (w_i+h) \sign(S_i)+\beta \sum_{j=-K}^{K}\sum_{i=1}^{N} \gamma_{i}^{j} \boldsymbol{1}_{\{S_{i}=j\}}$. This transformed path measure describes an hydrophobic(philic) copolymer interacting with a layer of width $2K$ around an interface between oil and water. In the present article we prove the convergence in the limit of weak coupling (when $\lambda$, $h$ and $\beta$ tend to 0) of this discrete model towards its continuous counterpart. To that aim we further develop a technique of coarse graining introduced by Bolthausen and den Hollander in \cite{BDH}. Our result shows, in particular, that the randomness of the pinning around the interface vanishes as the coupling becomes weaker.