Time-of-Flight Detection of Boson Peak in Volcanic Glass

A universal excitation called a boson peak has been observed in glasses in the terahertz range. The terahertz time-domain spectroscopy (THz-TDS) enables the time-of-flight (TOF) detection of a boson peak. Volcanic obsidian is a typical natural inorganic glass. The broadband frequency dependence of complex dielectric constants was measured by TOF. The boson peak at 1.07 THz was observed at room temperature, which is lower than 1.2 THz of silica glass. The decrease in BP frequency may be attributed to the modulation of the SiO₄ network by metal ions in obsidian. However, the IR light-vibration coupling coefficient C_IR of obsidian is much higher than that of silica glass. The uncorrelated charge fluctuations, σ1=q1i2=0.481e were determined by C_IR in Taraskin's model. The value of obsidian is much higher than σ1 = 0.06e of silica glass. However, this value is comparable with σ1≈0.44e of sodosilicate glass, xNa₂O(1-x)SiO₂ at x = 0.2. As a common feature in silica-based glasses, the content of metal elements may lead to a nonlocal redistribution of all the atomic charges, which induces a weakening of Si-O bonds, and the uncorrelated charge fluctuations can be enhanced.