Effect of Disorder on Thermal Phases AssociatedWith Two-Dimensional Melting

Prashanti, Jami (2024) Effect of Disorder on Thermal Phases AssociatedWith Two-Dimensional Melting. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Jami Prashanti (17RS044)) 17RS044.pdf - Submitted Version Restricted to Repository staff only Download (26MB)

Abstract

The impact of quenched disorder on the properties of ordered systems, particularly in the context of a two-dimensional classical particle system undergoing melting, has been a subject of extensive investigation. This system exhibits distinct threshold temperatures for translational and bond-orientational order, resulting in a hexatic phase between the solid and liquid states in clean two-dimensional systems. Understanding the influence of disorder on this melting process and the fate of the hexatic phase has been a significant focus of research. In this thesis, we delve into the influence of disorder on two-dimensional thermal phases, considering confined and bulk systems. The thesis is divided into two parts. The first part explores the significance of disorder details on Coulomb clusters, employing Molecular Dynamics. We analyze the static and dynamic responses in a cluster of Coulomb particles, specifically studying melting in confined systems with pinned impurities and comparing the results with irregularly trapped systems. Disorder of the first type leads to diffusive single-particle dynamics, while motion in an irregular trap remains chaotic but ballistic. Systems with pinned impurities exhibit a more robust positional order than in irregular confinements, with similar bond orientational order responses. Although the two systems show different spatial ordering, the thermal crossover temperature remains consistent. The details of disorder play a minimal role in the thermal melting of the Coulomb“solid," yet quantitative differences emerge, notably in relaxation time scales tuned by impurity concentration. In the second part, we investigate melting in a two-dimensional system with Gaussian-core interactions amid quenched disorder. The clean system validates the conventional two-step melting wisdom, introducing a hexatic phase between solid and isotropic liquid. Pinning impurities alter this behavior significantly. A random distribution of pinning centers induces a low-temperature state with a crossover between an amorphous solid and hexatic phase, transitioning to a liquid at a single melting temperature. Conversely, pinning centers located at a randomly chosen fraction of perfect crystal sites anchor a low-temperature solid phase that encompasses the hexatic phase and undergoes a direct transition to the liquid. Despite distinct routes to melting depending on disorder nature, both cases lose the celebrated two-step melting, exhibiting only a singlestep transition.

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