Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

Yusuke Nambu, Jason S. Gardner, Douglas E. MacLaughlin, Chris Stock, Hitoshi Endo, Seth Jonas, Taku J. Sato, Satoru Nakatsuji, and Collin Broholm

Phys. Rev. Lett. 115, 127202 – Published 15 September 2015

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 $4 K < T \leq T^{*}$ . 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 ${NiGa}_{2} S_{4}$ .

Received 4 September 2014

DOI:https://doi.org/10.1103/PhysRevLett.115.127202

Authors & Affiliations

Yusuke Nambu^1,2,*, Jason S. Gardner^3,4, Douglas E. MacLaughlin⁵, Chris Stock⁶, Hitoshi Endo^7,†, Seth Jonas⁸, Taku J. Sato¹, Satoru Nakatsuji^9,10, and Collin Broholm^8,3

¹Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan
²Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan
³NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
⁴National Synchrotron Radiation Research Center, Neutron Group, Hsinchu 30077, Taiwan
⁵Department of Physics and Astronomy, University of California, Riverside, California 92521, USA
⁶School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom
⁷Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
⁸Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
⁹Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
¹⁰PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan

^*Present address: Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan.
^†Present address: Neutron Science Laboratory, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan and Department of Material Structure Science, The Graduate University for Advanced Studies (SOKENDAI), 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan.