Physics of phonon-polaritons in amorphous materials

The nature of bosonic excitations in disordered materials has remained elusive due to the difficulties defining key concepts such as quasi-particles presence disorder. We report on experimental observation phonon-polaritons glasses, including a boson peak (BP), i.e. excess THz modes over Debye law. A theoretical framework based concept diffusons is developed model broadening linewidth polariton disorder-induced scattering. It shown that scaling BP frequency with diffusion constant strongly correlates Ioffe-Regel (IR) crossover polariton. This result demonstrates universality low-energy spectra collective bosonic-like well beyond traditional case acoustic phonons, and establishes IR fundamental physical mechanism behind BP.