Neutron scattering and thermodynamic evidence for emergent photons and fractionalization in a pyrochlore spin ice

The three-dimensional pyrochlore lattice of corner-sharing tetrahedra can host a quantum spin ice, a quantum analogue of the classical spin ice found in other pyrochlore compounds. This state can manifest a quantum spin liquid, and indeed, these compounds are predicted to have emergent gauge fields that produce linearly dispersing collective magnetic excitations near zero energy, in addition to the presence of higher-energy spinon excitations. Here we use polarized neutron scattering experiments on single crystals of the Ce₂Zr₂O₇ pyrochlore. We find evidence for magnetic excitations near zero energy, in addition to signatures of spinons at higher energies. Furthermore, we perform heat capacity measurements and find behaviour consistent with the cubic-in-temperature dependence expected for linearly dispersing gapless bosonic modes. Comparing the observed magnetic excitations with theoretical calculations, we argue that Ce₂Zr₂O₇ is a strong candidate for a dipolar–octupolar quantum spin ice with dominant dipolar Ising interactions.