Emergent photons and fractionalized excitations in a quantum spin liquid

A quantum spin liquid (QSL) arises from a highly entangled superposition of many degenerate classical ground states in a frustrated magnet, and is characterized by emergent gauge fields and deconfined fractionalized excitations (spinons). Because such a novel phase of matter is relevant to high-transition-temperature superconductivity and quantum computation, the microscopic understanding of QSL states is a long-sought goal in condensed matter physics. Although Kitaev QSL exists in an exactly solvable spin-1/2 () model on a two-dimensional (2D) honeycomb lattice, there is currently no conclusive identification of a Kitaev QSL material. The 3D pyrochlore lattice of corner-sharing tetrahedra, on the other hand, can host a QSL with U(1) gauge fields called quantum spin ice (QSI), which is a quantum (with effective ) analog of the classical (with large effective moment) spin ice. The key difference between a QSI and classical spin ice is the predicted presence of the linearly dispersing collective excitations near zero energy, dubbed the “photons” arising from emergent quantum electrodynamics, in addition to the spinons at higher energies. Recently, 3D pyrochlore systems Ce₂M₂O₇ (M = Sn, Zr, Hf) have been suggested as effective  QSI candidates, but there has been no evidence of quasielastic magnetic scattering signals from photons, a key signature for a QSI. Here, we use polarized neutron scattering experiments on single crystals of Ce₂Zr₂O₇ to conclusively demonstrate the presence of magnetic excitations near zero energy at 50 mK in addition to the signatures of spinons at higher energies. By comparing the energy (E), wave vector (), and polarization dependence of the magnetic excitations with theoretical calculations, we conclude that Ce₂Zr₂O₇ is the first example of a dipolar-octupolar π-flux QSI with dominant dipolar Ising interactions, therefore identifying a microscopic Hamiltonian responsible for a QSL.