Observation of topological Z2 vortex fluctuations in the frustrated Heisenberg magnet NaCrO2

Abstract

Spin fluctuations in the triangular-lattice Heisenberg antiferromagnet ${\mathrm{NaCrO}}_{2}$ are investigated by means of quasielastic neutron scattering with high energy resolution and a 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 $\ensuremath{\sim}0.001{E}_{\mathrm{ex}}$ (${E}_{\mathrm{ex}}$ is the exchange energy), nearly disappears at low temperature (10 K), and the other, with an extended lifetime of $\ensuremath{\sim}0.01{E}_{\mathrm{ex}}$, survives there, identified as the free ${\mathbb{Z}}_{2}$ vortex and ${\mathbb{Z}}_{2}$-vortex pair, respectively, in agreement with the ${\mathbb{Z}}_{2}$-vortex theory.