Unjamming in models with analytic pairwise potentials

TL;DR

This study investigates unjamming in systems with smooth, non-analytic pairwise potentials, demonstrating that key unjamming features persist and enabling quantitative proximity estimates without a sharp coordination number.

Contribution

It extends unjamming analysis to models with analytic potentials, establishing relations between pressure, bulk modulus, and unjamming, and broadening applicability to generic glass formers.

Findings

Unjamming features appear in systems with smooth potentials.

Pressure to bulk modulus ratio relates to the distance from unjamming.

Nonaffine contributions are irrelevant for bulk modulus near unjamming.

Abstract

The canonical models for studying the unjamming scenario in systems of soft repulsive particles assume pairwise potentials with a sharp cut-off in the interaction range. The sharp cut-off renders the potential non-analytic, but makes it possible to describe many properties of the solid in terms of the coordination number $z$, which has an unambiguous definition in these cases. Pairwise potentials without a sharp cut-off in the interaction range have not been considered in this context, but are of interest for understanding the relevance of the unjamming phenomenology to systems in which such a cut-off cannot be assumed. In this work we explore two systems with such interactions: an inverse power law and an exponentially decaying pairwise potential, with the control parameters being the exponent (of the inverse power-law) for the former and the number density for the latter. Both systems are shown to exhibit the characteristic features of the unjamming transition, among which are the vanishing of the shear to bulk modulus ratio and the emergence of an excess of low-frequency vibrational modes. We establish a relation between the hydrostatic pressure to bulk modulus ratio and the distance to unjamming in each of our model systems. This allows us to predict the dependence of other key observables on the distance to unjamming. Our results provide the means for a quantitative estimation of the proximity of generic glass forming models to the unjamming transition in the absence of a clear-cut definition of the coordination number, and highlight the general irrelevance of nonaffine contributions to the bulk modulus.