Seeing the light: Experimental signatures of emergent electromagnetism in a quantum spin ice

The "spin-ice" state found in the rare-earth pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 offers a beautiful realization of classical magnetostatics, complete with magnetic monopole excitations. It has been suggested that in "quantum spin-ice" materials, quantum-mechanical tunneling between different ice configurations, could convert the magnetostatics of spin ice into a quantum spin liquid that realizes a fully dynamical, lattice analogue of quantum electromagnetism. Here, we explore how such a state might manifest itself in experiment, within the minimal microscopic model of a such a quantum spin ice. We develop a lattice field theory for this model, and use this to make explicit predictions for the dynamical structure factor that would be observed in neutron scattering experiments on a quantum spin ice. We find that "pinch points," which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its zero-temperature ground state. We also make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the "photons" of this emergent electromagnetism. The predictions of this field theory are shown to be in quantitative agreement with quantum Monte Carlo simulations at zero temperature.