title: Suppression of tunneling two-level systems in ultrastable glasses of indomethacin.
abstract: Glasses and other noncrystalline 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 kelvin, 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 [Yu CC, Leggett AJ (1988) Comments Condens Matter Phys 14(4):231-251; Leggett AJ, Vural DC (2013) J Phys Chem B 117(42):12966-12971] to the standard tunneling model, although more experiments in different kinds of ultrastable glasses are needed to ascertain this hypothesis.
authors.affiliation: Laboratorio de Bajas Temperaturas, Departamento de Física de la Materia Condensada, Condensed Matter Physics Center, and Instituto Nicolás Cabrera, Universidad Autónoma de Madrid, E-28049 Madrid, Spain;
authors.forename: Tomás
authors.initials: T
authors.lastname: Pérez-Castañeda
authors.affiliation: Grupo de Nanomateriales y Microsistemas, Departamento de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; and.
authors.forename: Cristian
authors.initials: C
authors.lastname: Rodríguez-Tinoco
authors.affiliation: Grupo de Nanomateriales y Microsistemas, Departamento de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; andMATGAS Research Center, 08193 Bellaterra, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodríguez-Viejo
authors.affiliation: Laboratorio de Bajas Temperaturas, Departamento de Física de la Materia Condensada, Condensed Matter Physics Center, and Instituto Nicolás Cabrera, Universidad Autónoma de Madrid, E-28049 Madrid, Spain; miguel.ramos@uam.es.
authors.forename: Miguel A
authors.identifier: http://orcid.org/0000-0002-2157-9774
authors.initials: MA
authors.lastname: Ramos
keywords: glass transition; glassy anomalies; low-temperature properties of glasses; organic glasses