Spin liquids, which are in the focus of modern solid state research, are exotic ground states of spin systems, which are governed by strong magnetic exchange, but due to frustration effects undergo no long-range spin order down to the lowest temperatures. More than 10 years ago, our group, Experimental Physics V, presented structural, magnetic susceptibility and heat-capacity results, presenting two strongly frustrated magnets, namely MnSc2S4 and FeSc2S4. Both compounds crystallize in the normal AB2X4 spinel structure, with magnetic ions at the A-site only. The A site in the normal spinel structure corresponds to a diamond lattice, consisting of two interpenetrating fcc lattices. Depending on the strength of the competing interactions between and within these sublattices, long-range magnetic order can be fully suppressed. At that time we proposed that at low temperatures, the manganese compound realizes a spin-liquid, while the iron compound is one of the rare examples of a spin-orbital liquid ground state. Three years later, Leon Balents and coworkers from UCSB predicted a new spin-liquid state in MnSc2S4, a spiral spin-liquid in which the ground state is a massively degenerate set of coplanar spin spirals. These authors predicted that the spiral propagation vectors form a unique spiral surface of strong spin fluctuations in momentum space, which they proposed could be observed by neutron-scattering experiments. At that time, the experiment was impossible because there were no single crystals available. After 10 years of hard work, Vladimir Tsurkan of Experimental Physics V in Augsburg, succeeded in growing single crystals, large enough to perform quasi-elastic neutron scattering. Now neutron-scattering experiments have been performed at the Heinz Maier-Leibnitz Zentrum in Munich, as well as at the spallation neutron source SINQ of the Paul Scherrer Institute in Villingen (CH) by Christian Rüegg and coworkers, and one main result is shown in the figure and compared to the theoretical predictions. The figure clearly shows the spiral surface – or rather a cut through it, which results in a rounded square even in the raw data.
Figure: (c) Diffuse scattering intensities in MnSc2S4 in the (HK0) plane measured at 2.9 K compared to (d) Monte-Carlo simulations for spin-spiral correlations in the (HK0) plane, using the J1-J2 model of Balents and coworkers.
|||V. Fritsch, J. Hemberger, N. Büttgen, E.-W. Scheidt, H.-A. Krug von Nidda, A. Loidl, and V. Tsurkan, Phys. Rev. Lett. 92, 116401 (2004).|
|||D. Bergman, J. Alicea, E. Gull, S. Trebst, and L. Balents, Nature Physics 3, 487 (2007).|
|||S. Gao, O. Zaharko, V. Tsurkan, Y. Su, J. S. White, G. S. Tucker, B. Roessli, F. Bourdarot, R. Sibille, D. Chernyhov, T. Fennel, A. Loidl, and Ch. Rüegg, Nature Physics, October 24, 2016, AOP|