Discovery of Emergent Photon and Monopoles in a Quantum Spin Liquid

TL;DR

This paper reports the discovery of emergent photons and monopoles in a quantum spin liquid, evidenced by unusual heat conduction and highly mobile spin excitations in a pyrochlore magnet, revealing new quasiparticle phenomena.

Contribution

It provides experimental evidence for emergent photons and monopoles in a quantum spin liquid, advancing understanding of fractionalization and topological order in such states.

Findings

Observation of two-gap thermal conductivity behavior.

Detection of highly mobile spin excitations below 200 mK.

Identification of emergent photons as gapless spin excitations.

Abstract

Quantum spin liquid (QSL) is an exotic quantum phase of matter whose ground state 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 of the pyrochlore magnet Pr$_2$Zr$_2$O$_7$, 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 temperatures below 200\,mK, the thermal conductivity unexpectedly shows a dramatic 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 ($\nu$-particle), coherent gapless spin excitations in a spin-ice manifold.