Suppression of tunneling two-level systems in ultrastable glasses of indomethacin

TL;DR

This study demonstrates the suppression of tunneling two-level systems in ultrastable indomethacin glasses, suggesting their layered structure influences low-energy defect interactions, challenging traditional tunneling models.

Contribution

It provides experimental evidence that ultrastable indomethacin glasses lack typical TLS, highlighting the role of anisotropic structure over thermodynamic stability in TLS suppression.

Findings

Ultrastable indomethacin glasses show no linear specific heat contribution from TLS.

Conversion to conventional glass restores typical TLS levels.

Layered and anisotropic structure likely hinders TLS formation.

Abstract

Glasses and other non-crystalline solids exhibit thermal and acoustic properties at low temperatures anomalously different from those found in crystalline solids, and with a remarkable degree of universality. Below a few K, these universal properties have been successfully interpreted using the Tunneling Model, which has enjoyed (almost) unanimous recognition for decades. Here we present low-temperature specific-heat measurements of ultrastable glasses of indomethacin that clearly show the disappearance of the ubiquitous linear contribution traditionally ascribed to the existence of tunneling two-level systems (TLS). When the ultrastable thin-film sample is thermally converted into a conventional glass, the material recovers a typical amount of TLS. This remarkable suppression of the TLS found in ultrastable glasses of indomethacin is argued to be due to their particular anisotropic and layered character, which strongly influences the dynamical network and may hinder isotropic interactions among low-energy defects, rather than to the thermodynamic stabilization itself. This explanation may lend support to the criticisms by Leggett and others to the standard Tunneling Model, although more experiments in different kinds of ultrastable glasses are needed to ascertain this hypothesis.