Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu_3Zn(OH)_6FBr

We report a new kagome quantum spin liquid candidate Cu_3Zn(OH)_6FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (\sim200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from ^19F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.