Density of Quasi-localized Modes in Glasses: where are the Two-Level Systems?

TL;DR

This study investigates the nature of quasi-localized modes in glasses and questions whether two-level systems are necessary to explain the universal density of states observed at ultra-low temperatures.

Contribution

It identifies two distinct types of quasi-localized modes with different frequency dependencies and analyzes the glass energy landscape to explain their origins, challenging the role of two-level systems.

Findings

Two types of QLMs with different density of states identified

The properties of the energy landscape explain the creation of these modes

TLS may not be necessary for the universal density of QLMs

Abstract

The existence of a constant density of two-level systems (TLS) was proposed as the basis of some intriguing universal aspects of glasses at ultra-low temperatures. Here we ask whether their existence is necessary for explaining the universal density of states quasi-localized modes (QLM) in glasses at ultra-low temperatures. A careful examination of the QLM that exist in a generic atomistic model of a glass former reveals at least two types of them, each exhibiting a different density of states, one depending on the frequency as $\omega^3$ and the other as $\omega^4$. The properties of the glassy energy landscape that is responsible for the two types of modes is examined here, explaining the analytic feature responsible for the creations of (at least) two families of QLM's. Although adjacent wells certainly exist in the complex energy landscape of glasses, doubt is cast on the relevance of TLS for the universal density of QLM's.