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Antiferromagnetism, spin liquid, and plaquette singlet phases in the Shastry-Sutherland model: A variational cluster approximation approach

Jean Paul Latyr Faye1,*, Oumar Ndiaye1, and David Sénéchal2

  • *Contact author: jeanpaullatyr.faye@ucad.edu.sn
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Phys. Rev. B 114, 074406 – Published 5 August, 2026

DOI: https://doi.org/10.1103/fmrx-4pxw

Abstract

We study the half-filled single-band Hubbard model on the Shastry-Sutherland lattice with nearest-neighbor hopping t, incorporating (i) a frustrating diagonal hopping t′ to probe the emergence of spin-liquid behavior, and (ii) an additional Heisenberg exchange J that explicitly stabilizes plaquette-singlet correlations. Using the variational cluster approximation (VCA) with exact diagonalization at zero temperature on an eight-site cluster, we map the phase diagrams in the (U/t,t′/t) and (U/t,J/t) parameter planes. In the (U/t,t′/t) plane, our analysis uncovers a competition between Néel antiferromagnetic (AFM) order with staggered magnetization and a nonmagnetic regime lacking conventional order, consistent with a nonmagnetic insulating phase that may be related to the spin-liquid-like regimes reported in previous studies. In contrast, the (U/t,J/t) phase diagram exhibits a competition between the Néel antiferromagnet and a solid valence-bonding valence bond plaquette (PS), where spins form localized singlet plaquettes. Most notably, we observe that the transition from the PS phase to the Néel AFM phase is a first-order phase transition, with a discontinuous jump of the order parameters at the phase boundary, in quantitative agreement with high-pressure heat-capacity measurements on SrCu2(BO3)2 [J. Guo et al., Commun. Phys. 8, 75 (2025)]. Thus, our VCA analysis underscores the subtle interplay of geometric frustration, quantum fluctuations, and dimensionality, and elucidates the crucial role of short-range correlations in stabilizing and competing ground states of frustrated quantum magnets.

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Phys. Rev. B 114, 074406– Published 5 August, 2026
  • Received 9 April 2026
  • Revised 21 June 2026
  • Accepted 22 July 2026
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