Structure and melting behavior of classical bilayer crystals of dipoles

TL;DR

This paper investigates the structure and melting behavior of a classical bilayer system of dipoles, revealing a ground state of two hexagonal crystals and a non-monotonic melting temperature influenced by interlayer spacing.

Contribution

It provides a detailed analysis of the ground-state configurations and melting transitions of dipolar bilayer crystals, including the discovery of re-entrant melting behavior.

Findings

Ground state is two hexagonal crystals aligned on top of each other.

Melting temperature varies non-monotonically with interlayer distance.

Re-entrant solid-liquid-solid-liquid transitions occur at fixed temperature.

Abstract

We study the structure and melting of a classical bilayer system of dipoles, in a setup where the dipoles are oriented perpendicular to the planes of the layers and the density of dipoles is the same in each layer. Due to the anisotropic character of the dipole-dipole interactions, we find that the ground-state configuration is given by two hexagonal crystals positioned on top of each other, independent of the interlayer spacing and dipolar density. For large interlayer distances these crystals are independent, while in the opposite limit of small interlayer distances the system behaves as a two-dimensional crystal of paired dipoles. Within the harmonic approximation for the phonon excitations, the melting temperature of these crystalline configurations displays a non-monotonic dependence on the interlayer distance, which is associated with a re-entrant melting behavior in the form of solid-liquid-solid-liquid transitions at fixed temperature.