The Shastry-Sutherland model and its realization in SrCu₂(BO₃)₂: spin liquid phase and deconfined quantum criticality.

The Shastry-Sutherland model (SSM) [1] dates back to the early 1980s and is a corner stone of frustrated quantum magnetism. For small ratios g=J/J' of the inter- to intra-dimer couplings, the ground state is an exact product of dimer singlets. The SSM has a close realization in SrCu₂(BO₃)₂, and many experiments have shown that, indeed, the ground state is a gapped dimer singlet state with properties matching the model with g=0.63. Numerical studies (e.g., [2]) have shown that the ground state of the SSM changes by a level crossing at g=0.68 into a doubly-degenerate plaquette-singlet (PS) ordered state. It was believed that this type of ground state persists until a Neel antiferromagnetuc state forms at g=0.76. However, recent work indicates that there is also a gapless spin liquid phase between the PS and Neel states [3], and that this spin liquid may be connected to a so-called deconfined quantum critical point, beyond which the PS-N'eel transition is direct. In SrCu₂(BO₃)₂, the coupling ratio g can be substantially increased under pressure, enough so to access the PS and Neel states (and possibly the putative spin liquid, though experiments at low enough temperatures in the corresponding pressure regime are difficult). Recent high-pressure NMR experiments [4] show a direct PS-Neel transition versus an external magnetic field, and scaling behavior of the spin-lattice relaxation rate indicates proximity to a deconfined quantum critical point. I will review the current status of spin liquid and deconfined quantum criticality, in the SSM as well as in SrCu₂(BO₃)₂.

[1] B. S. Shastry and B. Sutherland, Physica B+C 108, 1069 (1981).

[2] P. Corboz and F. Mila, Phys. Rev. B 87, 115144 (2013).

[3] J. Yang, A. W. Sandvik, and L. Wang, Phys. Rev. B 105, L060409 (2022).

[4] Y. Cui et al., Science 380, 1179 (2023).