Structure and dynamics of quantum supercooled liquids:Insights from molecular dynamics simulations

Abstract

Supercooled liquids are formed when a liquid is cooled below the melting temperature, avoiding crystallization. The system gets trapped in one of the many metastable states in the free energy surface. In supercooled state, the dynamics of the system shows marked differences compared to the normal liquid state. Upon decreasing the temperature further, molecular motion becomes so slow that it cannot be identified in experimentally accessible timescales, with no significant changes in the liquid structure. The dynamic slow-down in supercooled liquids arises from the collective behavior or “caging” of particles by their neighbors. The thesis begins with a discussion of cage dynamics in a classical 2D model liquid. The cage time diverges as a power-law, which is much slower than the exponential divergence of the structural relaxation time. The cage sizes are correlated with the cage dynamics: small changes in the cage sizes in the deeply supercooled regime show large changes in the dynamics. Quantum effects may lead to counterintuitive behavior in the properties of liquids in the supercooled regime. The effects of quantum fluctuations on the dynamical and structural properties of supercooled liquids are studied in a simple atomistic model system, using molecular dynamics simulations. The quantum effects are accounted for using the quantum-classical ring polymer isomorphism. The interplay of the classical confining potential and the quantum tunneling brings in interesting effects. The effects of quantumness on the cages are analyzed. Dynamic heterogeneity is a hallmark of supercooled liquids. The qualitative changes in the dynamic heterogeneity due to quantum effects are studied by analyzing the tagged particle dynamics. In contrast to the classical case, quantum liquids show large dynamic heterogeneities at short timescales. Finally, the manifestations of quantum effects in supercooled water are studied. Molecular systems have additional degrees of freedom as compared to the atomistic system, which can give rise to more complex dynamics. The inclusion of quantum nature affects the structure and dynamics of water in the low-temperature regime. The dynamics is discussed in terms of the neighbor shells of a molecule, which shows some interesting features that arise due to inter-shell migration of molecules.