In our recent preprint we develop a microscopic theory of monopole motion in spin ice. There is ample experimental evidence for the existence of these three-dimensional fractionalized excitations but their dynamics remain poorly understood. Our theory starts from the observation that spins near a monopole, unlike other spins in the sample, experience transverse magnetic and exchange fields. These fields induce unitary oscillations of such spins, which gives those specific spins the ability to flip. This, in turn, induces hopping of the monopole.

Our theory is quantitative and microscopic and yields two distinct flipping rates (resulting from a bimodal distribution of dipolar and exchange fields). These appear to be in broad agreement with experiments. Our paper thus presents a very intriguing picture of spin ice as a largely classical material whose excitations, nevertheless, create quantum fluctuations in their immediate vicinity that are responsible for their ability to propagate.

The paper also contains some speculation, backed up by a remarkably simple calculation, on the possible role played by decoherence or even “measurement” in converting a unitary (quantum) evolution of individual spins into actual motion of a classical object (the monopole). This suggests spin ices might offer a novel setting in which to study the quantum-classical boundary.

Here is the abstract and link to the preprint on the arXiv server:

Correlated Quantum Tunnelling of Monopoles in Spin Ice

arxiv.org:1810.11469

The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 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 Pr2Sn2O7 and Pr2Zr2O7, 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.