Charge and spin correlations in the Monopole Liquid

Abstract: A \emph{monopole liquid} is a magnetic charge-disordered spin system defined over an Ising pyrochlore lattice, with one single topological charge or \emph{monopole} in each tetrahedron. We define a simple model Hamiltonian for this system and compare its thermodynamics at zero magnetic field with that of spin ice ---a phase free of these charges. In spite of the liquid-like correlations between charges, we find that spins in the charged phase are uncorrelated at all temperatures, like in a perfect paramagnet. The addition of nearest neighbors interactions favoring neutral `2in-2out' excitations as a perturbation has a peculiar effect. While they decrease charge-charge correlations, new spin correlations resembling those in spin ice appear on increasing temperature. This helps us understand why dipolar correlations are observed in spin ices at unexpectedly high temperatures, and the major role of double excitations in erasing the Coulomb phase correlations. Ferromagnetic interactions strengthen the charges short range order and its associated spin correlations. Finally, we discuss how the monopole liquid can be related to other systems and materials where different phases of \emph{monopole matter} have been observed.