How Cooperative are the Dynamics in Tunneling Systems? A Computer Study for an Atomic Model Glass

TL;DR

This study uses computer simulations of a binary Lennard-Jones glass to analyze tunneling systems, revealing how local structure and atom type influence their cooperative behavior and low-temperature anomalies.

Contribution

It provides a detailed analysis of tunneling systems in a glass model, highlighting the role of local density anomalies and atom type in their formation and nature.

Findings

Two types of tunneling systems depend on atom size.

Local density anomalies are crucial for tunneling.

Differences between single-particle and collective tunneling.

Abstract

Via computer simulations of the standard binary Lennard-Jones glass former we have obtained in a systematic way a large set of close-by pairs of minima on the potential energy landscape, i.e. double-well potentials (DWP). We analyze this set of DWP in two directions. At low temperatures the symmetric DWP give rise to tunneling systems. We compare the resulting low-temperature anomalies with those, predicted by the standard tunneling model. Deviations can be traced back to the energy dependence of the relevant quantities like the number of tunneling systems. Furthermore we analyze the local structure around a DWP as well as the translational pattern during the transition between both minima. Local density anomalies are crucial for the formation of a tunneling system. Two very different kinds of tunneling systems are observed, depending on the type of atom (small or large) which forms the center of the tunneling system. In the first case the tunneling system can be interpreted as a single-particle motion, in the second case it is more collective.