Charge ordering in a pure spin model: dipolar spin-ice

TL;DR

This study investigates charge ordering phenomena in a pure spin model of dipolar spin-ice at fixed excitation densities, revealing complex phase diagrams and ordering transitions driven by monopolar excitations.

Contribution

It introduces a Monte Carlo approach fixing excitation density, enabling exploration of charge ordering and phase transitions in spin-ice models at low monopole densities.

Findings

Identification of charge ordering transitions analogous to Coulomb charges.

Discovery of complex phase diagram with order within charges and vacuum.

Equilibration at low temperatures due to fixed monopole density.

Abstract

We study the dipolar spin-ice model at fixed density of single excitations, $\rho$, using a Monte Carlo algorithm where processes of creation and annihilation of such excitations are banned. In the limit of $\rho$ going to zero, this model coincides with the usual dipolar spin-ice model at low temperatures, with the additional advantage that a negligible number of monopoles allows for equilibration even at the lowest temperatures. Thus, the transition to the ordered fundamental state found by Melko et al. in 2001 is reached using simple local spin flip dynamics. As the density is increased, the monopolar nature of the excitations becomes apparent: the system shows a rich $\rho$ vs. $T$ phase diagram with "charge" ordering transitions analogous to that observed for Coulomb charges in lattices. A further layer of complexity is revealed by the existence of order both within the charges and their associated vacuum, which can only be described in terms of spins --the true microscopic degrees of freedom of the system.