Observation of topological $Z_2$ vortex fluctuations in the frustrated Heisenberg magnet NaCrO$_2$

Spin fluctuations in the triangular-lattice Heisenberg antiferromagnet NaCrO 2 are investigated by means of quasi-elastic neutron scattering with high energy resolution and wide energy band. Two components with the following features are captured separately. They are pronounced at intermediate temperatures of 20-50 K. One with quite an extended lifetime corresponding to ∼ 0 . 001 E ex ( E ex the exchange energy) nearly disappears at low temperature 10 K, and the other with an extended lifetime ∼ 0 . 01 E ex survives there, identified as free Z 2 vortex and Z 2 -vortex pair, respectively, in harmony with the Z 2 vortex theory.