Experimental evidence for the ω⁴ tail of the nonphononic spectra of glasses

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 D G ( ω ) ∼ ω 4 at low frequencies ω . Yet, due to intrinsic difficulties in disentangling D G ( ω ) from the total VDoS D ( ω ) , which is experimentally accessible through various scattering techniques, the ω 4 tail of D G ( ω ) lacked direct experimental support. We develop a procedure to extract D G ( ω ) from the measured D ( ω ) , based on recent advances in understanding low-frequency excitations in glasses, and apply it to available datasets for diverse glasses. The resulting analysis indicates that the ω 4 tail of the nonphononic vibrational spectra of glasses is nontrivially consistent with a broad range of experimental observations. It also further supports that D G ( ω ) makes an additive contribution to D ( ω ) .