Dynamics of Glass Forming Liquids with Emphasis on Vibrational Modes

Abstract

The potential energy landscape (PEL) description is regarded as one of the fundamental landmarks in the theory of glass transition, which has found wide applications not only in the understanding of disordered materials but also in other frontiers of research, such as bio-molecules, catalysis, machine learning, and neural networks. In this context, the mechanically stable local minimum energy configurations of the PEL, which are known as inherent structures (ISs), have a major role to play in elucidating various dynamical and thermodynamic properties of the system in terms of its sojourn over the complex multi-dimensional potential energy surface (PES). However contrary to the crystals, disordered materials pose a greater challenge in understanding their properties. Due to their ordered structures, the normal modes of collective vibrational excitations in crystals can be described by plane waves, which are known as phonons, and they play a fundamental role in determining the kinetic and thermodynamic properties of crystalline solids. Notwithstanding the difficulties presented by the inherent disorder in glass-forming materials, collective excitations associated with small-amplitude vibrations of the system about their ISs can be defined. This thesis presents a study of the dynamics of glass-forming liquids with emphasis on the collective vibrational excitations of the underlying disordered solid. The vibrational spectrum of disordered harmonic solids consists of a coexisting region of extended phonon modes that obey the Debye scaling law in their density of states, g(and#969;) and#8764; and#969;^(dand#8722;1) in d dimensions, and quasi-localized modes that obey a universal g(and#969;) and#8764; and#969;^4 law. These excess quasi-localized modes, appearing in the boson peak region of the vibrational spectrum, are found to be substantially different in nature as compared to the modes in other parts of the spectrum. The first part of the thesis describes an investigation of the dynamics of model glass-forming systems based on the measurement of thermal transport...