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An 'invisible order' in glass shapes vibrations in the terahertz frequency range

Although glasses exhibit disordered atomic structures, X-ray and neutron scattering reveal a subtle periodicity. Researchers at the University of Tsukuba have demonstrated that this hidden periodicity—referred to as "invisible order"—plays a critical role in determining vibrational fluctuations in the terahertz (THz) frequency range, which significantly influence the physical properties of glass.

The research is in the journal Scientific Reports.

At first glance, glass appears to be a random network of atoms. However, X-ray and neutron beam analysis reveals a faint but consistent periodic feature known as the first sharp diffraction peak (FSDP).

Concurrently, glass exhibits a boson peak (BP), a vibrational anomaly in the THz range, which contributes to its low thermal conductivity, mechanical characteristics, and THz range light absorption. Despite extensive research, the precise relationship between the FSDP and BP has remained unclear.

Using heterogeneous elasticity theory—which accounts for spatial fluctuations in the material's elastic properties—the researchers identified a direct correlation between the emergence of the BP and the presence of the FSDP.

The theoretical model indicates that the scale of elastic inhomogeneity necessary for BP manifestation aligns with that of the FSDP. This suggests that the FSDP is a determining factor in the vibrational behavior of glasses within the THz band.

These findings are expected to inform the development of novel glass materials with tunable boson peaks, enabling targeted control of their thermal and mechanical properties.