Part 1*

Ce₂Zr₂O₇ (CZO) and the isostructural Ce₂Hf₂O₇, Ce₂Sn₂O₇  have been identified as candidates for realizing Quantum Spin Liquid (QSL) and have been studied by heat capacity, magnetic susceptibility, muon spin relaxation, and inelastic neutron scattering (INS) measurements. The spinon continuum, the key signature of QSL, has been obtained by subtracting the high-temperature background from the low-temperature data in INS. However, this operation leaves no intensity at the quasi-elastic line, where the artificial “photon”, linearly dispersing transverse excitations of the gauge field A(r) with a speed of light c, was predicted in this lattice analog of quantum electrodynamics (QED). By careful polarized neutron analysis at the same low temperature, we extracted the magnetic signal in the quasi-elastic line for the first time, which was overwhelmed by the much larger background in previous INS experiments. Surprisingly, the magnetic signals in the quasi-elastic line are much larger than the previously observed spinon continuum and consist of multiple peaks. Our collaborators theoretically investigate the potential explanations using gauge mean-field theory (GMFT) and Gaussian QED to model the spinons and emergent photons. The fitting parameters put CZO in the region of π-flux QSL. This lends further overwhelming support to identifying CZO as a long-sought-after experimental realization of QSL.