Concentration and fluctuation phenomena in the localized phase of the pinning model

TL;DR

This paper investigates the localized phase of pinning models with i.i.d. disorder, providing detailed regularity estimates of the free energy and establishing a quenched CLT for the contact number, revealing disorder-dependent fluctuations.

Contribution

It develops quantitative correlation estimates and Gevrey-3 regularity results for the free energy, extending prior work to more general disorder distributions and explicit bounds.

Findings

Gevrey-3 regularity of free energy in the localized phase

Quenched CLT for contact number with disorder-dependent centering

Disorder fluctuations match thermal fluctuations in scale

Abstract

We focus on the localized phase of pinning models with i.i.d. site disorder on which we assume only that the moment generating function is bounded in a neighborhood of the origin. We develop quantitative correlation functions estimates for local observables that entail quantitative $C^\infty$ estimates on the free energy density, showing in particular that its regularity class is at least Gevrey-3 in the whole localized phase. We then explain how a quenched concentration bound and the quenched Central Limit Theorem (CLT) on the number of the pinned sites, i.e., the 'contact number', can be extracted from the regularity estimates on the free energy: this identifies the thermal fluctuations of the contact number. But the centering sequence in the quenched CLT is random in the sense that it is disorder dependent: we show that the (disorder induced) fluctuations of the centering are on the same scale of the thermal fluctuations by establishing a CLT, with a non degenerate variance, also for the centering. For what concerns the correlation and $C^\infty$ estimates, our work substantially generalizes and expands the analysis in [Giacomin and F. L. Toninelli, Lat. Am. J. Probab. 1 (2006), 149-180] that dealt with pinning models with restrictive conditions on the disorder distributions and in which less explicit, non uniform bounds were obtained.