报告摘要： The vibrational dynamics of glasses and crystals are remarkably similar, even though their low temperature thermal properties differ significantly. Differences between the ordered and the disordered states of matter appear only in the low frequency part of the vibrational spectrum, typically at and below the terahertz range [1]. Amorphous solids exhibit an enhanced density of vibrational states at the boson peak, the presence of Rayleigh scattering of sound waves [2] and relaxation processes that remain active well below the glass transition temperature and give sizeable contributions to sound attenuation and to light, x-rays and neutron scattering cross sections in the quasi-elastic regime.

In this seminar I will present recent experimental advancements on the investigation of the vibrational properties of glasses in the sub-terahertz frequency range. In a first part of the seminar, I will present results on thermal transport and on the sound propagation and damping in vitreous silica probed by means of transient grating spectroscopy with extreme UV radiation. This non-linear optical method allowed us to probe the thermoelastic properties of thin silica membranes in the frequency range between 70 and 370 GHz [3, 4], confirming the relevance of Rayleigh scattering of sound and the significant contribution of structural relaxation and an-harmonicity.

In a second part of the seminar, I will present results obtained by means of an innovative spectrograph for nuclear resonance analysis of inelastic x-ray scattering, that allowed us to probe the density of vibrational states with the exceptional energy resolution of 0.1 meV [5]. With the help of this instrument, we have investigated the vibrations of vitreous silica in a frequency range where anharmonic and relaxational processes give sizeable contributions to the scattering cross-section and the low frequency vibrations of an ultra-stable glass of TPD. I will discuss the relevance of quasi-localized vibrational modes in these two samples and the effect of glass stability on the low frequency vibrations.

[1] G. Baldi, A. Fontana, and G. Monaco, “Vibrational dynamics of non-crystalline solids”, Ed. M. A. Ramos, World Scientific, pag. 177 – 226 (2022).
[2] G. Baldi, et al., Phys. Rev. Lett. 104, 195501 (2010).
[3] D. Fainozzi, et al., APL Materials 12, 051122 (2024).
[4] D. Nabari et al., Phys. Rev. B (submitted).
[5] I. Festi, et al., Phys. Rev. X 16, 021021 (2026).

报告人简介：Giacomo Baldi is an Associate Professor of Physics at the University of Trento, Italy. Since 2020 he is the responsible of the “Structure and Dynamics of Complex Systems” group of the Physics Department. His research activity is focused on the physics of disordered materials and of soft matter (liquids, supercooled liquids, glasses, amorphous solids, colloids, and polymers), explored with various spectroscopic methods, ranging from X-ray based techniques to laser spectroscopies.

邀请人：罗鹏

联系人：赵睿

联系电话：82649267