4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2

Authors: M. E. Zayed, Ch. Rüegg, J. Larrea J., A. M. Läuchli, Christos Panagopoulos , S. S. Saxena, M. Ellerby, D. F. McMorrow, Th. Strässle, S. Klotz, G. Hamel, R. A. Sadykov, V. Pomjakushin, M. Boehm, M. Jiménez–Ruiz, A. Schneidewind, E. Pomjakushina, M. Stingaciu, K. Conder & H. M. Rønnow

Published in: Nature Physics , 2017.

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 states. The frustrated two-dimensional Shastry–Sutherland lattice realized by SrCu2(BO3)2  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 debated. 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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