Boson Peak and Fracton of Polymethyl Methacrylate Detected by Terahertz Time-Domain and Low-Frequency Raman Spectroscopies

摘要

Terahertz time-domain spectroscopy and low-frequency Raman scattering were performed on polymethyl methacrylate to detect the boson peak (BP) and fracton, which are universal excitations of glass. In infrared spectrum, BP is observed in the $lpha(v)/v^{2}$ plot, where $lpha(v)$ is absorption coefficient. Log-log plot of $lpha(v), vchi^{primeprime}(v)$ and the vibrational density of states show linear frequency dependence above the BP frequency, where $chi^{primeprime}(v)$ is the imaginary part of Raman susceptibility. We determined the infrared light vibration coupling constant $C_{ext{IR}}(v)$ and Raman coupling constant $C_{ext{Raman}}(v)$ to investigate how the fractal dimension and the fracton dimension appear in $C_{mathrm{R}}(v)$ and $C_{ext{Raman}}(v)$.