Classical, Quantum and Artificial Spin Ice

We propose a readily achievable experimental setting in which an external magnetic field is used to tune the emergent quantum electrodynamics (eQED) of dipolar-octupolar quantum spin ice (DO-QSI). In U(1)_π DO-QSI – the proposed ground state of highly promising QSI candidates Ce₂Zr₂O₇, Ce₂Sn₂O₇ and Ce₂Hf₂O₇ – we show that the field can be used to control the emergent speed of light (and, consequently, the emergent fine structure constant). Depending on the field’s alignment with the crystal, one may induce different speeds for the two polarizations of the emergent photons, in a fascinating analogue of the electro-optic Kerr effect. In strong fields, we find a number of unusual field-induced transitions, including a transition between π- and 0-flux QSI phases. Even more unusually, we find a field-induced phase with an unquantized, continuously tunable flux configuration analogous to magnetic monopoles in an electric field. We discuss experimental signatures of these effects in the spinon excitation spectrum, which can be readily accessed for instance by inelastic neutron scattering measurements. Our proposal opens the gate to a plethora of experimentally accessible, engineerable eQED phenomena in the emergent universes of quantum spin ice.