It is now established that glasses feature low-frequency, nonphononic excitations, in addition to phonons that follow Debye's vibrational density of state (VDoS). Extensive computer studies demonstrated that these nonphononic, glassy excitations follow a universal non-Debye VDoS ${ \cal D}_{ \rm G}(\omega)\!\sim\!\omega^4$, at low frequencies $\omega$. Yet, due to intrinsic difficulties in disentangling ${ \cal D}_{ \rm G}(\omega)$ from the total VDoS ${ \cal D}(\omega)$, which is experimentally accessible through various scattering techniques, the $\omega^4$ tail of ${ \cal D}_{ \rm G}(\omega)$ lacked direct experimental support. We develop a procedure to extract ${ \cal D}_{ \rm G}(\omega)$ from the measured ${ \cal D}(\omega)$, based on recent advances in understanding low-frequency excitations in glasses, and apply it to available datasets for diverse glasses. The resulting analysis shows that the $\omega^4$ tail of the nonphononic vibrational spectra of glasses is nontrivially consistent with a broad range of experimental observations. It also further supports that ${ \cal D}_{ \rm G}(\omega)$ makes an additive contribution to ${ \cal D}(\omega)$.
Categories: cond-mat.dis-nn cond-mat.mtrl-sci cond-mat.soft