Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

TL;DR

This study identifies specific structural motifs in amorphous alumina that act as tunneling two-level systems, providing atomistic insights into its low-temperature thermal anomalies.

Contribution

The paper uncovers previously unknown bistable structural motifs in amorphous alumina that can be modeled as TLSs, advancing understanding of its low-temperature properties.

Findings

Discovered new bistable structural motifs via molecular dynamics.

Predicted TLS properties consistent with experimental data.

Reduced complex energy landscape to canonical TLS models.

Abstract

One of the most accepted models that describe the anomalous thermal behavior of amorphous materials at temperatures below 1 K relies on the quantum mechanical tunneling of atoms between two nearly equivalent potential energy wells forming a two-level system (TLS). Indirect evidence for TLSs is widely available. However, the atomistic structure of these TLSs remains an unsolved topic in the physics of amorphous materials. Here, using classical molecular dynamics, we found several hitherto unknown bistable structural motifs that may be key to understanding the anomalous thermal properties of amorphous alumina at low temperatures. We show through free energy profiles that the complex potential energy surface can be reduced to canonical TLSs. The predicted tunnel splittings from instanton theory, the number density, dipole moment, and coupling to external strain of the discovered motifs are consistent with experiments.