Monopole matter from magnetoelastic coupling in the Ising pyrochlore

Abstract: Ising models on a pyrochlore oxide lattice are usually associated with spin ice materials and "magnetic monopoles". Ever more often effects connecting magnetic and elastic degrees of freedom are reported on these and other related frustrated materials. Here we extend a spin-ice Hamiltonian to include coupling between spins and the O$^{-2}$ ions mediating superexchange; we call it the Magnetoelastic Spin Ice model (MeSI). There has been a long search for a model in which monopoles would spontaneously become the building blocks of new ground-states: the MeSI Hamiltonian is such a model. In spite of its simplicity and classical approach, it describes (both spin and oxygen lattice) the double-layered monopole crystal observed in Tb$_2$Ti$_2$O$_7$. Remarkably, the dipolar electric moment of single monopoles emerges as a probe for magnetism. As an example we show that, in principle, pinch points related to Coulomb phases could be detected in association with the O$^{-2}$-ion displacements.