The Origin of the Boson Peak and the Thermal Conductivity Plateau in Low Temperature Glasses

TL;DR

This paper links the boson peak and thermal conductivity plateau in low-temperature glasses to glassy degrees of freedom associated with anharmonic transitions and domain wall motions, providing a theoretical explanation consistent with experimental observations.

Contribution

It introduces a model connecting low-temperature vibrational features to the glass transition and mosaic structure, explaining the non-universality of thermal conductivity in glasses.

Findings

The vibrational spectrum depends on $T_g$, Debye frequency, and molecular length scale.

The model reproduces the experimental boson peak.

The non-universality of the thermal conductivity plateau arises from interactions with phonons.

Abstract

We argue that the intrinsic glassy degrees of freedom in amorphous solids giving rise to the thermal conductivity plateau and the ``boson peak'' in the heat capacity at moderately low temperatures are directly connected to those motions giving rise to the two-level like excitations seen at still lower temperatures. These degrees of freedom can be thought of as strongly anharmonic transitions between the local minima of the glassy energy landscape that are accompanied by ripplon-like domain wall motions of the glassy mosaic structure predicted to occur at $T_g$ by the random first order transition theory. The energy spectrum of the vibrations of the mosaic depends on the glass transition temperature, the Debye frequency and the molecular length scale. The resulting spectrum reproduces the experimental low temperature Boson peak. The ``non-universality'' of the thermal conductivity plateau depends on $k_B T_g/\hbar \omega_D$ and arises from calculable interactions with the phonons.