Depletion of two-level systems in ultrastable computer-generated glasses

TL;DR

This study uses advanced simulations to show that increasing the stability of ultrastable glasses reduces the density of low-energy tunneling defects, aligning with experimental observations and revealing their localized nature.

Contribution

It introduces a novel computational approach to identify and analyze tunneling defects in glasses, demonstrating how stability affects defect density and localization.

Findings

Higher stability correlates with fewer tunneling defects.

Tunneling defects are mostly localized but can be delocalized.

Simulation results agree with recent experimental data.

Abstract

Amorphous solids exhibit quasi-universal low-temperature anomalies whose origin has been ascribed to localized tunneling defects. Using an advanced Monte Carlo procedure, we create {\it in silico} glasses spanning from hyperquenched to ultrastable glasses. Using a multidimensional path-finding protocol, we locate tunneling defects with energy splittings smaller than $k_{B}T_Q$, with $T_Q$ the temperature below which quantum effects are relevant ($T_Q \approx 1 \,$K in most experiments). We find that as the stability of a glass increases, its energy landscape as well as the manner in which it is probed tend to deplete the density of tunneling defects, as observed in recent experiments. We explore the real-space nature of tunneling defects, and find that they are mostly localized to a few atoms, but are occasionally dramatically delocalized.