Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

Phys Rev Lett. 2015 Sep 18;115(12):127202. doi: 10.1103/PhysRevLett.115.127202. Epub 2015 Sep 15.

Abstract

The temporal magnetic correlations of the triangular-lattice antiferromagnet NiGa_{2}S_{4} are examined through 13 decades (10^{-13}-1 sec) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards T^{*}=8.5 K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4 K<T≤T^{*}. As this time scale exceeds that of single-site dynamics by 6 orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of NiGa_{2}S_{4}.