Higher-Magnesium-Doping\nEffects on the Singlet Ground\nState of the Shastry–Sutherland SrCu<sub>2</sub>(BO<sub>3</sub>)<sub>2</sub>

Doping of quantum antiferromagnets is an established\napproach to\ninvestigate the robustness of their ground state against the competing\nphases. Predictions of doping effects on the ground state of the Shastry–Sutherland\ndimer model are here verified experimentally on Mg-doped SrCu<sub>2</sub>(BO<sub>3</sub>)<sub>2</sub>. A partial incorporation of Mg<sup>2+</sup> on the Cu<sup>2+</sup> site in the SrCu<sub>2</sub>(BO<sub>3</sub>)<sub>2</sub> structure leads to a subtle but systematic lattice\nexpansion with the increasing Mg-doping concentration, which is accompanied\nby a slight decrease in the spin gap, the Curie–Weiss temperature,\nand the peak temperature of the susceptibility. These findings indicate\na doping-induced breaking of Cu<sup>2+</sup> spin-1/2 dimers that\nis also corroborated by X-band EPR spectroscopy that points to a systematic\nincrease in the intensity of free Cu<sup>2+</sup> sites with increasing\nMg-doping concentration. Extending the Mg-doping up to nominal <i>x</i> = 0.10 yielding SrCu<sub>1.9</sub>Mg<sub>0.1</sub>(BO<sub>3</sub>)<sub>2</sub>, in the magnetization measurements taken up\nto 35 T, a suppression of the pseudo-1/8 plateau is found along\nwith a clear presence of an anomaly at an\nonset critical field μ<sub>0</sub><i>H’</i><sub>C0</sub> ≈ 9 T. The latter, absent in pure SrCu<sub>2</sub>(BO<sub>3</sub>)<sub>2</sub>, emerges due to the pairwise coupling\nof liberated Cu<sup>2+</sup> spin-1/2 entities in the vicinity of\nMg-doping induced impurities.