The Effect of Disorder on Polymer Depinning Transitions

Abstract

We consider a polymer, with monomer locations modeled by the trajectory of a Markov chain, in the presence of a potential that interacts with the polymer when it visits a particular site 0. We assume that probability of an excursion of length $n$ is given by $n^{-c}ϕ(n)$ for some $1<c<2$ and slowly varying $ϕ$. Disorder is introduced by having the interaction vary from one monomer to another, as a constant $u$ plus i.i.d. mean-0 randomness. There is a critical value of $u$ above which the polymer is pinned, placing a positive fraction (called the contact fraction) of its monomers at 0 with high probability. To see the effect of disorder on the depinning transition, we compare the contact fraction and free energy (as functions of $u$) to the corresponding annealed system. We show that for $c>3/2$, at high temperature, the quenched and annealed curves differ significantly only in a very small neighborhood of the critical point--the size of this neighborhood scales as $β^{1/(2c-3)}$ where $β$ is the inverse temperature. For $c<3/2$, given $ε>0$, for sufficiently high temperature the quenched and annealed curves are within a factor of $1-ε$ for all $u$ near the critical point; in particular the quenched and annealed critical points are equal. For $c=3/2$ the regime depends on the slowly varying function $ϕ$.