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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Affiliations

  1. Laboratory for Quantum Magnetism, Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland

    • M. E. Zayed
    • , J. Larrea J.
    •  & H. M. Rønnow
  2. Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

    • M. E. Zayed
    • , Ch. Rüegg
    • , Th. Strässle
    •  & V. Pomjakushin
  3. Department of Physics, Carnegie Mellon University in Qatar, Education City, PO Box 24866, Doha, Qatar

    • M. E. Zayed
  4. Department of Quantum Matter Physics, University of Geneva, 1211 Geneva 4, Switzerland

    • Ch. Rüegg
  5. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK

    • Ch. Rüegg
    • , M. Ellerby
    •  & D. F. McMorrow
  6. Centro Brasileiro de Pesquisas Fisicas, Rua Doutor Xavier Sigaud 150, CEP 2290-180, Rio de Janeiro, Brazil

    • J. Larrea J.
  7. Institut für Theoretische Physik, Universität Innsbruck, 6020 Innsbruck, Austria

    • A. M. Läuchli
  8. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK

    • C. Panagopoulos
    •  & S. S. Saxena
  9. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

    • C. Panagopoulos
  10. IMPMC; CNRS–UMR 7590, Université Pierre et Marie Curie, 75252 Paris, France

    • S. Klotz
    •  & G. Hamel
  11. Institute for Nuclear Research, Russian Academy of Sciences, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia

    • R. A. Sadykov
  12. Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, 108840, Moscow, Troitsk, Russia

    • R. A. Sadykov
  13. Institut Laue-Langevin, 71 avenue des Martyrs - CS 20156- 38042 Grenoble Cedex 9, France

    • M. Boehm
    •  & M. Jiménez–Ruiz
  14. Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany

    • A. Schneidewind
  15. Laboratory for Scientific Developments and Novel Materials, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

    • E. Pomjakushina
    • , M. Stingaciu
    •  & K. Conder

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Contributions

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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to M. E. Zayed.

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https://doi.org/10.1038/nphys4190

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