Resumen de Low-temperature specific heat of hyperaged and ultrastable glasses
Amorphous solids present a large number of differences with their crystalline counterparts which undoubtedly constitute a great challenge for the physicists dealing with glass forming systems. Since the entropy crisis was first stated by Kauzmann in 1948, open questions on the phenomenology of supercooled liquids and glasses have steadily increased. More than twenty years later, the discovery of their anomalous behavior compared to the Debye prediction in the low-temperature thermal properties, observed by Zeller and Pohl in 1971, meant a starting point in the race to understand the new phenomena hidden in disordered matter. Moreover, the universality of these low-temperature anomalies among the amorphous solids strongly demanded explanation. Only one year later this observation Anderson, Halperin and Varma and Phillips, independently gave an explanation for the excess density of states in amorphous solids below 1 K introducing the concept of the tunneling states. Although it successfully accounted for the deviations from the Debye-model predictions below 1 K, it failed in the understanding of the plateau in thermal conductivity and the maximum in the reduced specific heat representation CP/T3 at temperatures 2 K ¿ T ¿ 10 K, typically. The so called boson peak is still in the present a topic of intense debate in the scientific community due to the lack of consensus on the origin of this excess in the Vibrational Density of States. Comprehension on the microscopic nature of the boson peak is needed, what will help us defining the key ingredients present in all amorphous solids which are responsible of their universal properties.
The access to this microscopic understanding is however arduous. The non-stable thermodynamic character of disordered systems further increases the difficulty to access a conclusion: it introduces the evolution with time to the physics of glasses. Many attempts have been done to identify the manner in which intermolecular forces originate the complex behavior in supercooled liquids and glasses: varying the route to obtain the glass, changing the thermal history, the composition, using polymorphism and polyamorphism¿ Despite the exhaustive studies carried out in an endless list of non-crystalline solids in the last forty years, many of the findings done are far from being definite, and therefore susceptible to be interpreted in complete opposite directions depending on the theoretical view defended.
The application of extreme physical processes on glass forming systems and amorphous solids provides an extraordinary possibility to explore regions of the potential energy landscape never accessed before. This would shed light on the validity of the existing models and theories, which perhaps could have been developed and supported on the base of experimental observations far from being general.
The practical access to extreme amorphous solids has been conducted in this thesis in two different ways. The original idea we have pursued is studying glasses which have suffered an extraordinary stabilization process as a result of ageing. Given the geological character of amber, the well-known natural resin, which has stood the test of time for periods of several tens of millions or even over a hundred million years, is an unbeatable candidate to study the effects of extreme ageing or hyperageing in glasses. It gives us the chance to study for the first time the combined effects of extreme stabilization in the glass transition phenomenology and in the low-temperature universal anomalies of glasses.
In order to study the phenomenology of extremely stabilized glasses, two calorimetric techniques have been employed, the Differential Scanning Calorimetry and the low-temperature relaxation calorimetry. In this thesis we have also designed and built a versatile calorimeter for the low-temperature measurements which has allowed us to access the specific heat of glasses ranging from 50 mK up to 40 K or above. The elastic and acoustic properties of the glasses studied here have been also determined using a complementary technique like Brillouin scattering.
The access to the stability reached in amber glasses in laboratory time scales has only been possible in recent years with the discovery of ultrastable thin-film glasses grown by vapor deposition. This has given us the chance to study a second system with extraordinary stability but which involves a completely different route to obtain it. The determination of the specific heat of indomethacin ultrastable glasses at very low temperatures done in this thesis entails the first approach to the study of the universal glassy anomalies of ultrastable thin films.
The joint research done in these two glassy systems which present the highest stability reached up to the date, and at the same time have quite different nature, leads to a new understanding of the microscopic origin of the excess density of states present in amorphous solids.
