Abstract
The study of interacting spin systems is of fundamental importance for modern condensed-matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states1. The frustrated two-dimensional Shastry–Sutherland lattice2 realized by SrCu2(BO3)2 (refs 3,4) is an important test case for our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated2,4,5,6,7,8,9,10,11,12,13,14. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu2(BO3)2. It is observed by inelastic neutron scattering after pressure tuning to 21.5 kbar. This gapped singlet state leads to a transition to long-range antiferromagnetic order above 40 kbar, consistent with the existence of a deconfined quantum critical point.
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Acknowledgements
We thank A. Magee for her contributions to the susceptibility measurements, M. Merlini and M. Hanfland for support during high-pressure X-ray diffraction experiments at the ESRF, and M. Ay and P. Link for assistance during neutron scattering experiments. We acknowledge F. Mila and B. Normand for many useful discussions. We also thank CamCool Research Ltd for supplying the pressure cells for the SQUID measurements. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland, at the FRM-2, Munich, Germany, and at the ILL, Grenoble, France. We thank the Swiss National Science Foundation SNF and the Royal Society (UK) for financial support. The work in London and Cambridge was supported by the EPSRC. J.L.J. acknowledges the Science Without Borders program of CNPq/MCTI-Brazil and C.P. acknowledges financial support from the National Research Foundation (NRF) of Singapore, through NRF Investigatorship (Reference No. NRF-NRFI2015-04).
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M.E.Z., C.R. and H.M.R. designed the research, performed the experiments and analysed the data. A.M.L. computed the magnetic susceptibility by exact diagonalization. C.P., S.S.S. and M.E. helped with susceptibility experiments. T.S., S.K., G.H. and R.A.S. provided neutron high-pressure techniques. M.B., M.J.-R., A.S., V.P. and T.S. provided support for neutron experiments. E.P., M.S. and K.C. synthesized the SrCu2(BO3)2 samples. J.L.J. and D.F.M. contributed to interpretation of the data. M.E.Z., C.R. and H.M.R. wrote the manuscript with contributions from all co-authors.
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Zayed, M., Rüegg, C., Larrea J., J. et al. 4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2. Nature Phys 13, 962–966 (2017). https://doi.org/10.1038/nphys4190
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DOI: https://doi.org/10.1038/nphys4190
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