Discovery of Emergent Photon and Monopolies in a Quantum Spin Liquid

Abstract

Quantum spin liquid (QSL) is an exotic quantum phase of matter whose ground stale is quantum-mechanically entangled without any magnetic ordering. A central issue concerns emergent excitations that characterize QSLs, which are hypothetically associated with quasiparticle fractionalization and topological order. Here we report highly unusual heat conduction generated by the spin degrees of freedom in a QSL state ol the pyrochlore magnet Pr2Zr2O7, which hosts spin-ice correlations with strong quantum fluctuations. The thermal conductivity in high temperature regime exhibits a two-gap behavior, which is consistent with the gapped excitations of magnetic (M-) and electric monopoles (E-particles). At very low temperatuies below 200 mK, the thermal conductivity unexpectedly shows a diamatic enhancement, which well exceeds purely phononic conductivity, demonstrating the presence of highly mobile spin excitations. This new type of excitations can be attributed to emergent photons (v-particle), coherent gapless spin excitations in a spin-ice manifold.