Low-frequency vibrational spectrum of mean-field disordered systems

TL;DR

This paper analyzes a mean-field disordered model to understand the vibrational density of states, revealing a transition from a gapped to a gapless spectrum with different localization properties at the glass transition.

Contribution

It introduces an exactly solvable mean-field model showing both quadratic delocalized and quartic localized vibrational spectra at the glass transition.

Findings

Gapped spectrum for small coupling J when p(κ) is gapped.

Transition to gapless spectrum at a critical line in (h,J) phase diagram.

Emergence of a quartic localized pseudogap at high field h.

Abstract

We study a recently introduced and exactly solvable mean-field model for the density of vibrational states $\mathcal{D}(\omega)$ of a structurally disordered system. The model is formulated as a collection of disordered anharmonic oscillators, with random stiffness $\kappa$ drawn from a distribution $p(\kappa)$, subjected to a constant field $h$ and interacting bilinearly with a coupling of strength $J$. We investigate the vibrational properties of its ground state at zero temperature. When $p(\kappa)$ is gapped, the emergent $\mathcal{D}(\omega)$ is also gapped, for small $J$. Upon increasing $J$, the gap vanishes on a critical line in the $(h,J)$ phase diagram, whereupon replica symmetry is broken. At small $h$, the form of this pseudogap is quadratic, $\mathcal{D}(\omega)\sim\omega^2$, and its modes are delocalized, as expected from previously investigated mean-field spin glass models. However, we determine that for large enough $h$, a quartic pseudogap $\mathcal{D}(\omega)\sim\omega^4$, populated by localized modes, emerges, the two regimes being separated by a special point on the critical line. We thus uncover that mean-field disordered systems can generically display both a quadratic-delocalized and a quartic-localized spectrum at the glass transition.