Topics in dynamics, thermodynamics and electronic structure of supercooled liquids

Abstract

This is a synopsis of the thesis entitled "Topics in Dynamics, Thermodynamics and Electronic Structure of Supercooled Liquids.", delivered by Ashwin S. Sampangiraj of the Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. The thesis is divided into the following five parts. • The dynamical behaviour of glass-forming liquids have been analyzed extensively via computer simulations of model liquids, among which the Kob-Andersen binary Lennard-Jones mixture has been a widely studied system. Typically, studies of this model have been restricted to temperatures above the mode coupling temperature. Preliminary results concerning the dynamics of the Kob-Andersen binary mixture are presented at temperatures that extend below the mode coupling temperature, along with properties of the local energy minima sampled. We show that a crossover in the dynamics occurs, alongside changes in the properties of the local energy minima sampled, from non-Arrhenius behaviour of the diffusivity above the mode coupling temperature, to Arrhenius behaviour at lower temperatures. Computer simulations, using the Stillinger-Weber potential, have previously been employed to demonstrate a liquid-liquid transition in supercooled silicon near 1060 K. Prom calculations of electronic structure using an empirical psuedopotential, we show that silicon undergoes an associated metal to semi-metal transition with a resistivity jump of roughly one order of magnitude. We show that the electronic states near the Fermi energy become localized in the low temperature phase, and that changes in electronic structure between the two phases arise from a change in atomic structure, and not from a change in density. We also investigate the electronic structure of the quenched structures in these two phases. • We investigate the mechanical properties of several model liquids and corresponding local energy minima (inherent structures) in an attempt to explore their connection to slow dynamics and vitrification. In particular, we study the correlation between the distribution of forces between particles and the approach to the glass transition in a variety of liquids (with both attractive and repulsive interactions), including network forming liquid silicon, and silica. Such a correlation has been proposed, in analogy with granular materials, within the framework of a unified "jamming phase diagram" and has been studied for some model liquids through simulations recently. We postulate that the plateau behaviour at low forces is related to the fragility of the glass former, and provide preliminary supporting evidence. We also consider the critical strain amplitude needed to cause inherent structure transitions. and show that the critical strain correlates with the depth of the local energy minima and the onset of slow dynamics. • The stability of a liquid is bounded by the liquid-gas spinodal and the ideal glass transition. We calculate these stability boundaries using the Mezard-Parisi method for evaluating the thermodynamic glass transition and the Zerah-Hansen scheme for the equation of state of a model liquid. These two limiting boundaries intersect at a finite temperature. Our calculations are consistent with results from a previous work based on computer simulations. • Using Miiller-Plathe's method for calculating viscosity in computer simulations, we demonstrate the break down of the Stokes-Einstein relation between a liquid's viscosity and diffusivity. We calculate the viscosity and the diffusivity at various temperatures across the onset temperature of slow dynamics. We verify that the hydrodynamic radius does not remain a constant below the onset temperature. This is a signature of the break down of the Stokes-Einstein relation.