Localized low-frequency vibrational modes in glasses

Using a larger data set and more detailed analysis, we continue our examination of the zero-temperature vibrational spectrum of a glass of soft spheres by computer simulation. As in our previous study [Phys. Rev. Lett. 66, 636 (1991)], a normal-mode analysis shows clearly the existence of (quasi-)localized modes at the low-frequency end of the vibrational spectrum. The modes are found to be localized around atoms whose neighborhood structure differs significantly from the average glass environment and is reminiscent of that associated with interstitial defects in crystals. The effective masses of these modes range upward from 10 atomic masses.