Correlated Quantum Tunnelling of Monopoles in Spin Ice.

The spin ice materials Ho$_{2}$Ti$_{2}$O$_{7}$ and Dy$_{2}$Ti$_{2}$O$_{7}$ are by now perhaps the best-studied classical frustrated magnets. A crucial step towards the understanding of their low temperature behaviour -- both regarding their unusual dynamical properties and the possibility of observing their quantum coherent time evolution -- is a quantitative understanding of the spin-flip processes which underpin the hopping of magnetic monopoles. We attack this problem in the framework of a quantum treatment of a single ion subject to the crystal, exchange and dipolar fields from neighbouring ions. By studying the fundamental quantum mechanical mechanisms, we discover a bimodal distribution of hopping rates which depends on the local spin configuration, in broad agreement with rates extracted from experiment. Applying the same analysis to Pr$_{2}$Sn$_{2}$O$_{7}$ and Pr$_{2}$Zr$_{2}$O$_{7}$, we find an even more pronounced separation of time scales signalling the likelihood of coherent many-body dynamics that is likely to have signatures in the behaviour of these systems.