Researchers from UAB, MATGAS and UAM have discovered that ultrastable glasses of indomethacin are a surprising exception to the ubiquitous presence of quantum tunneling states at cryogenic temperatures in amorphous solids.

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

Glasses are disordered solids usually obtained by supercooling a liquid bypassing crystallization. A remarkable feature of glasses is that, independently of their nature and composition, they exhibit universal properties in the low-temperature range. Of interest here, the specific heat is characterized by a linear term below 1K, ascribed to quantum tunneling between two states of similar energy, known as Two Levels Systems, TLS. This model named Tunneling Model, TM, has gained popularity since its development in 1972 and is currently accepted by a majority of scientist in the field. Only very few authors, including Physics Nobel prize Anthony Legget, posed criticisms against the standard TM, pointing out how improbable it was that a random ensemble of independent tunneling states would produce essentially the same universal properties for all glasses, independently of their nature.

A collaboration between researchers from the Physics Department of the Universitat Autònoma de Barcelona (UAB), Cristian Rodríguez Tinoco and Javier Rodríguez Viejo (UAB-MATGAS), and from the Condensed Matter Physics Department of the Universidad Autónoma de Madrid (UAM), Tomás Pérez Castañeda and Miguel Ángel Ramos, have investigated if this ubiquitous behavior also applies to so-called ultrastable glasses, directly synthesized from the vapor phase into low-energy positions of the potential-energy landscape. Interestingly, the authors find a full suppression of the linear term in the specific heat breaking the universality of this property. In addition, when the sample absorbs water or when it is prepared directly from the liquid the presence of TLS reappears. This finding questions the current view of the popular tunneling model and sheds light on the microscopic origin of two-level systems in glasses, since the suppression is thought to be related to the anisotropic character of the ultrastable IMC glass that modifies the interaction between neighboring molecules. Introduction of water restores the molecular interaction.

Ultrastable glasses represent one of the most intriguing phenomena in recent glass research. They are synthesized by physical vapor-deposition in short-time scales at temperatures slightly below the glass transition temperature and show unprecedented thermodynamic and kinetic stability. A glass produced from the liquid will need millions of years to reach the same level of stability as an ultrastable glass produced in the lab in few hours. These glasses, discovered in 2007 by the group of Prof. Ediger from the University of Wisconsinn, Madison, are the purpose of intense research in the glass community. Their exceptional stability influence many of the striking properties of these glasses, among them the differentiated mechanism by which they transform into the liquid. When compared to conventional glasses prepared from the liquid these glasses also show higher density, higher sound velocity, higher moduli, they absorb less water, and many other properties that could impact their use for new applications. Some researchers argue that these glasses may provide the key to disentangle some of the unknowns of the glassy state. In particular, the possible existence of a first order transition to a glassy ground state that is typically hidden by the molecular arrest that occurs at the glass transition temperature when the liquid is cooled down.

Scheme of the preparation and growth of thin films of indomethacin.(a) The anisotropy of the ultrastable glass blocks the interaction between the two level systems (b) This mechanism is restored upon water absorption when the film partially losses its ultrastable character. Ultrastable glasse are grown at 0.1 nm/s with a substrate temperatura of 0.85 Tg, the glass transition temperature.

Scheme of the energy wells associated to the two level systems, together with the IMC molecule. The graph shows the suppression of the TLS in the ultrastable glass and its recovery after water absorption.

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