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. Three components of spin fluctuations, which are different in temperature dependence and time scale, are captured. The slowest component is described by a very sharp quasi-elastic scattering with energy width of approximately 0.01 meV ($\sim 0.001J$, where $J$ the exchange coupling), which is significant in the middle-temperature range of 30-40 K and goes away both in the lower and higher temperature ranges. The experimental features are checked against the $Z_2$ vortex theory to arrive at a natural explanation, indicating that the elusive $Z_2$ vortex fluctuations are finally detected directly.