Low-temperature specific heat of compacted vitreous silica

Universal behavior of low-temperature heat capacity of acrylonitrile-butadiene-styrene thermoplastic polymer and its composite with graphene oxide

The low-temperature dependence of the heat capacity of acrylonitrile-butadiene-styrene (ABS) polymer and its composite with thermally reduced graphene oxide was studied. The existence of a so-called “boson peak” characteristic of orientational and structural glasses was demonstrated. The boson peak appears in the form of a local maximum in the heat capacity curve displayed as C/T3 vs T at Tmax = 3.52 K. It was found that for both ABS polymer and its composite, as well as for a number of other substances of a crystalline and amorphous nature, the manifestation of the anomaly of the boson peak in the heat capacity has a universal character that is described by an empirical function Δ*. The value of Δ* depends on the magnitude of the anomaly in the heat capacity and the temperature of the boson peak manifestation. Thus, this study provides new physical information about the possible causes of the boson peak appearance in disordered materials and indicates the universality of boson peak anomaly for substances with short- and long-range order.

Tracking the Connection between Disorder and Energy Landscape in Glasses Using Geologically Hyperaged Amber

Fossil amber offers the unique opportunity to investigate an amorphous material that has been exploring its energy landscape for more than 110 million years of natural aging. By applying different X-ray scattering methods to amber before and after annealing the sample to erase its thermal history, we identify a link between the potential energy landscape and the structural and vibrational properties of glasses. We find that hyperaging induces a depletion of the vibrational density of states in the terahertz region, also ruling the sound dispersion and attenuation properties of the corresponding acoustic waves. Critically, this is accompanied by a densification with structural implications different in nature from that caused by hydrostatic compression. Our results, rationalized within the framework of fluctuating elasticity theory, reveal how upon approaching the bottom of the potential energy landscape (9% decrease in the fictive temperature) the elastic matrix becomes increasingly less disordered (6%) and longer-range correlated (22%).

Hydrogen-free amorphous silicon with no tunneling states

The ubiquitous low-energy excitations, known as two-level tunneling systems (TLSs), are one of the universal phenomena of amorphous solids. Low temperature elastic measurements show that e-beam amorphous silicon (a-Si) contains a variable density of TLSs which diminishes as the growth temperature reaches 400 °C. Structural analyses show that these a-Si films become denser and more structurally ordered. We conclude that the enhanced surface energetics at a high growth temperature improved the amorphous structural network of e-beam a-Si and removed TLSs. This work obviates the role hydrogen was previously thought to play in removing TLSs in the hydrogenated form of a-Si and suggests it is possible to prepare "perfect" amorphous solids with "crystal-like" properties for applications.

Influence of packing on low energy vibrations of densified glasses

A comparative study of Raman scattering and low temperature specific heat capacity has been performed on samples of B2O3, which have been high-pressure quenched to go through different glassy phases having growing density to the crystalline state. It has revealed that the excess volume characterizing the glassy networks favors the formation of specific glassy structural units, the boroxol rings, which produce the boson peak, a broad band of low energy vibrational states. The decrease of boroxol rings with increasing pressure of synthesis is associated with the progressive depression of the excess low energy vibrations until their full disappearance in the crystalline phase, where the rings are missing. These observations prove that the additional soft vibrations in glasses arise from specific units whose formation is made possible by the poor atomic packing of the network.

What is moving in silica at 1 K? A computer study of the low-temperature anomalies

Though the existence of two-level systems (TLS) is widely accepted to explain low-temperature anomalies in many physical observables, knowledge about their properties is very rare. For silica, which is one of the prototype glass-forming systems, we elucidate the properties of the TLS via computer simulations by applying a systematic search algorithm. We get specific information in the configuration space, i.e., about relevant energy scales, the absolute number of TLS, and electric dipole moments. Furthermore, important insight about the real-space realization of the TLS can be obtained. Comparison with experimental observations is included.