Detection of boson peak and fractal dynamics of disordered systems using terahertz spectroscopy
Tatsuya Mori, Yue Jiang, Yasuhiro Fujii, Suguru Kitani, Hideyuki Mizuno, Akitoshi Koreeda, Leona Motoji, Hiroko Tokoro, Kentaro Shiraki, Yohei Yamamoto, Seiji Kojima
Abstract
The boson peak is a largely unexplained excitation found universally in the terahertz vibrational spectra of disordered systems; the so-called fracton is a vibrational excitation associated with the self-similar structure of monomers in polymeric glasses. We demonstrate that such excitations can be detected using terahertz spectroscopy. In the case of fractal structures, we determine the infrared light-vibration coupling coefficient for the fracton region and show that information concerning the fractal and fracton dimensions appears in the exponent of the absorption coefficient. Finally, using terahertz time-domain spectroscopy and low-frequency Raman scattering, we experimentally observe these universal excitations in a protein (lysozyme) system that has an intrinsically disordered and fractal structure and argue that the system should be considered a single supramolecule. These findings are applicable to amorphous and fractal objects in general and will be valuable for understanding universal dynamics of disordered systems via terahertz light.