Abstract
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 towards understanding harmonic vibrational states. In particular, it has been established that quasi-localized vibrational modes emerge in addition to phonon-like 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 crystals, where particle rearrangements never occur. Remarkably, for both the phonon modes and quasi-localized modes, the vibrational modes exhibit strong anharmonicities, and the induced particle rearrangements are always localized in space and are composed of 1 to 1000 particles. Our findings contribute to the understanding of low-temperature thermal properties, for which anharmonic vibrations are crucial.
Highlights
Amorphous solids exhibit vibrational and thermal properties that are markedly different from those of crystals [1,2]
We attempt to address the following questions. (i) Can the vibrational modes exhibit anharmonicities that induce particle rearrangements and cause transitions to different states? (ii) If so, what are properties of the particle rearrangements and the transitions? (iii) What are the differences in the anharmonicities between phonon modes and quasilocalized vibrational (QLV) modes? We discuss two-level system (TLS) in relation to what we find regarding the anharmonicities of vibrational modes
This has already been observed for the harmonic potential system in previous work [50]; here, we confirmed the same result for the LJ potential system
Summary
Amorphous solids exhibit vibrational and thermal properties that are markedly different from those of crystals [1,2]. Anharmonicities become small or even negligible at low T and as a result, the specific heat can be well described by harmonic vibrations without anharmonicities [3,4] For amorphous solids, another type of anharmonicity has been proposed, the so-called two-level system (TLS) [21,22,23,24]. [40,41,42,43,44,45]) have attempted to detect the TLSs and clarify their nature, e.g., the statistics of parameters characterizing the TLSs (such as the distance and energy difference between two states), transition paths connecting two states, or the density of TLSs On another front, a recent work [46] demonstrated that the QLV modes exhibit unstable vibrations in a localized region.
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