Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses

Abstract

We present theoretical investigation on the high-frequency collective dynamics in liquids and glasses at microscopic length scales and terahertz frequency region based on the mode-coupling theory for ideal liquid-glass transition. We focus on recently investigated issues from inelastic-Xray-scattering and computer-simulation studies for dynamic structure factors and longitudinal and transversal current spectra: the anomalous dispersion of the high-frequency sound velocity and the nature of the low-frequency excitation called the boson peak. It will be discussed how the sound mode interferes with other low-lying modes present in the system. Thereby, we provide a systematic explanation of the anomalous sound-velocity dispersion in systems -ranging from high temperature liquid down to deep inside the glass state -in terms of the contributions from the structuralrelaxation processes and from vibrational excitations called the anomalous-oscillation peak (AOP). A possibility of observing negative dispersion -the decrease of the sound velocity upon increase of the wave number -is argued when the sound-velocity dispersion is dominated by the contribution from the vibrational dynamics. We also show that the low-frequency excitation, observable in both of the glass-state longitudinal and transversal current spectra at the same resonance frequency, is the manifestation of the AOP. As a consequence of the presence of the AOP in the transversal current spectra, it is predicted that the transversal sound velocity also exhibits the anomalous dispersion. These results of the theory are demonstrated for a model of the Lennard-Jones system.

Static heterogeneity in metallic glasses and its correlation to physical properties

“…It is observed in many disordered systems [12]. For liquids, positive dispersion may be explained within the framework of the mode coupling theory [13], in which a fast rattling process, distinct from α or β (JG) relaxation, is supposed to be present; see for details Ref. [14].…”

“…[14]. However, positive dispersion is also observed in rigid glasses [15][16][17], and the phenomenon has been attributed to residual fast process in the glass [12,13].…”

Inhomogeneity and glass-forming ability in the bulk metallic glassPd42.5Ni7.5Cu30PHosokawa1, Sato2, Ichitsubo3 et al. 2009Phys. Rev. B197282View full textAdd to dashboardCite

“…2 For liquids, the positive dispersion may be explained within the framework of generalized hydrodynamics 3 or mode-coupling theory. 4 However, positive dispersion is also observed in glasses, e.g., silica glass 5,6 and the phenomenon has been attributed to a residual fast process in the glass. 6 Ichitsubo et al 7 have recently carried out inelastic x-ray scattering ͑IXS͒ measurements on Pd 42.5 Ni 7.5 Cu 30 P 20 ͑PNCP͒.…”

Some comments on fluctuating-elasticity and local oscillator models for anomalous vibrational excitations in glasses

“…The models have been solved with the help of numerical simulations as well as effective-medium theories. ii) In theories of the glass transition [35][36][37][38][39] the boson peak arises as a benchmark of the frozen glassy state. iii) Defects with a heavy mass can produce resonant quasi-local resonant states within the DOS [40][41][42] and be thus the reason for the boson peak and the reduction of the thermal conductivity.…”