The Emergent Fine Structure Constant of Quantum Spin Ice Is Large

Condensed matter systems act as mini-universes with emergent low-energy properties drastically different from those of the standard model. A case in point is the emergent quantum electrodynamics (QED) in the fractionalized topological magnet known as quantum spin ice, whose magnetic monopoles set it apart from the familiar QED of the world we live in. Here, we show that the two greatly differ in their fine-structure constant α, which parametrizes how strongly matter couples to light: αQSI is more than an order of magnitude greater than αQED≈1/137. Furthermore, αQSI, the emergent speed of light, and all other parameters of the emergent QED, are tunable by engineering the microscopic Hamiltonian. We find that αQSI can be tuned all the way from zero up to what is believed to be the strongest possible coupling beyond which QED confines. The large αQSI implies that experiments probing candidate condensed-matter realizations of quantum spin ice should expect to observe phenomena arising due to strong interactions such as well-defined Coulomb bound states, Sommerfeld enhancement of particle pair creation, and copious emergent Cerenkov radiation. At finite temperature, the system further provides a platform for studying a strongly coupled electro-magnetic plasma.