Evidence for a Z2 topological ordered quantum spin liquid in a kagome-lattice antiferromagnet*

A quantum spin liquid with a Z2 topological order has long been thought to be important for the application of quantum computing and may be related to high-temperature superconductivity. While a two-dimensional kagome antiferromagnet may host such a state, strong experimental evidence is still lacking. Here we show that Cu3Zn(OH)6FBr exhibits gapped spin continuum at low temperature and is not magnetically ordered down to 20 mili Kelvin. The spin triplet gap value is about twice of the spinon gap value reported previously. Our results provide firm ground for the existence of spin-1/2 spinon excitations in Cu3Zn(OH)6FBr, whose ground state is thus a gapped quantum spin liquid with Z2 topological order.