Evidence for fractional matter coupled to an emergent gauge field in a quantum spin ice

Abstract

Electronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. They are expected to form in frustrated magnets that do not exhibit symmetry-breaking order down to zero temperature. Quantum spin ice is a theoretically well-established example described by an emergent quantum electrodynamics, with quasiparticle excitations behaving like photons and fractionally charged matter. However, in frustrated magnets it remains difficult to establish convincing experimental evidence for quantum spin liquid ground states and their fractional excitations. Here we study the time-dependent magnetic response of the candidate quantum spin ice material Ce₂Sn₂O₇. We find a gapped spectrum that features a threshold and peaks that match theories for pair production and propagation of fractional matter excitations strongly coupled to a background quantum electrodynamic field. The multiple peaks in our neutron spectroscopy data are a specific signature of the so-called π-flux phase of quantum spin ice, providing spectroscopic evidence for fractionalization in a three-dimensional quantum spin liquid.