Terahertz spectroscopic study on polymethyl methacrylate: Boson peak and fracton

For disordered systems, universal excitation called boson peak appears in the terahertz range. In addition, for polymeric glasses, fractal dynamics, so-called fracton, is expected to appear above the boson peak frequency as a result of self-similarity of monomer unit. In this study, we performed terahertz time-domain spectroscopy on polymethyl methacrylate to detect the boson peak and fracton. Obtained spectra are compared with results of low-frequency Raman scattering and low-temperature specific heat measurement. We also determined both the infrared light vibration and Raman coupling constants to investigate how the fractal and fracton dimensions appear in both coupling constants.