Mesoscopic and microscopic dipole clusters: Structure and phase transitions

TL;DR

This paper investigates the structure and phase transitions of 2D dipole particle clusters, revealing hierarchical melting processes in mesoscopic clusters and identifying stable configurations with maximal melting temperatures.

Contribution

It provides detailed analysis of ground states, normal modes, and melting behaviors of dipole clusters, highlighting size-dependent stability and phase transition mechanisms.

Findings

Hierarchical melting in mesoscopic clusters with N<37

Outer shell orientational melting in larger clusters

Stable clusters correspond to concentric hexagonal lattice shells

Abstract

Two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. For clusters of N < 81 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte Carlo and molecular dynamic methods are used to study in detail the order - disorder transition (the "melting" of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In "macroscopic" clusters (N > 37) an orientational "melting" of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are that of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The study of different quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between "solidlike" and "orientationally disordered" forms of clusters is considered.