'Boson peak' and high-frequency excitations in glassy crystals

TL;DR

This study investigates the nature of the boson peak in glassy crystals using dielectric loss spectroscopy, revealing it results from a mix of collective phonons and local modes, and identifies a fast process in these materials.

Contribution

It provides new experimental evidence that the boson peak arises from a combination of collective and localized excitations in plastic crystals.

Findings

Boson peak in plastic crystals differs from canonical glasses.

Mixing of phonons and local relaxational modes causes the boson peak.

A fast process is observed in plastic crystals.

Abstract

The high-frequency excitations in glasses and supercooled liquids belong to the great mysteries of the physics of condensed matter. While the fast process, located at GHz-THz, can be interpreted as encaged molecular motion, the occurrence of the boson peak (BP), dominating at THz frequencies, still is controversially debated. Two scenarios have evolved during recent years: the BP is truly collective in nature or, alternatively, it results from localized modes. To help solving this controversy, we have investigated the high-frequency dielectric loss of plastic crystals (PCs), having long-range translational but no orientational order. Having in mind the well-defined phonon modes in PCs compared to canonical glasses, and the infrared silence of acoustic phonon modes, millimeter-wave and far-infrared spectroscopy should provide valuable hints on the true origin of the BP. Here we show that PCs exhibit a BP-like feature with markedly different spectral shape compared to canonical glasses, evidencing that mixing of collective phonon with local relaxational modes causes the BP. In addition, we find clear evidence for a fast process also in PCs.