The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.

The soft potential model (an extension of the tunneling model to include soft localized vibrations) is shown to describe the anomalous features of the specific heat ${\mathit{C}}_{\mathit{P}}$ and the thermal conductivity of glasses over the entire low-temperature range, up to an including the peak in ${\mathit{C}}_{\mathit{P}}$/${\mathit{T}}^3$ and the second rise of the thermal conductivity above the plateau.

The microscopic dynamics of several phases of solid ethanol are studied under the same thermodynamic conditions by inelastic neutron scattering. It is found that the vibrational density of states of the orientational glass phase, where the molecules are arranged on an ordered lattice but with disordered orientations, is very similar to that of the structural glass phase, where the molecules are disordered both in position and orientation. Low-temperature specific heat measurements on the same phases strongly support the neutron measurements. We therefore find that positional disorder, even in a stoichiometrically homogeneous system such as ethanol, is not essential for the manifestation of glasslike behavior to an extent comparable with that exhibited by the structural glass.

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.