Anharmonic properties of vibrational excitations in amorphous solids

Understanding the vibrational and thermal properties of amorphous solids is one of the most discussed and long-standing issues in condensed matter physics. Recent works have made significant steps toward understanding harmonic vibrational states. In particular, it has been established that quasilocalized vibrational modes emerge in addition to phononlike vibrational modes. In this work, we study the anharmonic properties of these vibrational modes. We find that vibrational modes exhibit anharmonicities that induce particle rearrangements and cause transitions to different states. These anharmonicities are distinct from those in (perfect) crystals, where particle rearrangements never occur. Remarkably, for both the phonon modes and quasilocalized modes, the vibrational modes exhibit strong anharmonicities, and the induced particle rearrangements are always localized in space and composed of 1 to 1000 particles. Our findings contribute to the understanding of low-temperature thermal properties, for which anharmonic vibrations are crucial.