ABSTRACT
The nature of bosonic excitations in disordered materials has remained elusive due to the difficulties in defining key concepts such as quasi-particles in the presence of disorder. We report on an experimental observation of phonon-polaritons in glasses, including a prominent boson peak (BP), i.e., excess of THz modes over the Debye law. A theoretical framework based on the concept of diffusons is developed to describe the broadening linewidth of the polariton due to disorder-induced scattering. It is shown here for the first time that the BP frequency and the Ioffe–Regel (IR) crossover frequency of the polariton collapse onto the same power-law decay with the diffusivity of the bosonic excitation. This analysis dismisses the hypothesis of the BP being caused by a relic of the van Hove singularity. The presented framework establishes a new methodology to analyze bosonic excitations in amorphous media, well beyond the traditional case of acoustic phonons, and establishes the IR crossover as the fundamental physical mechanism behind the BP.
ACKNOWLEDGMENTS
M.B. acknowledges the support of the Spanish MINECO’s “Centro de Excelencia Severo Ochoa” program, under Grant No. SEV-2012-0249. A.Z. acknowledges financial support from the U.S. Army Research Laboratory and the U.S. Army Research Office, through Contract No. W911NF-19-2-0055. T.M. is grateful to Y. Matsuda for providing the glass sample and acknowledges JSPS KAKENHI, Grant Nos. JP17K14318 and JP18H04476, and the Asahi Glass Foundation.
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