The Shastry-Sutherland compound ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ has attracted considerable interest as a platform for exploring quantum phases and quantum phase transitions driven by magnetic frustration. The pressure-induced structural and magnetic phase transitions in ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$, however, remain controversial. To address this issue, we performed high-pressure ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) measurements on ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ up to 14 GPa. The NMR spectra reveal two pressure-induced monoclinic phases. With pressure above 4 GPa and with temperature below 10 K, the rapid broadening of the NMR spectrum and the power-law behavior of the spin-lattice relaxation rate $1/T_1$ provide clear evidence for a gapless 3D antiferromagnetic (AFM) phase in the monoclinic phase. At an intermediate temperature range around 20 K, the emergence of the field-dependent NMR line splitting resolves a two-dimensional, short-range ordered AFM phase; at temperature above 30 K, the sublinear power-law behavior of $1/T_1$ identifies an extended correlated paramagnetic regime.

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• Antiferromagnetism
• Frustrated magnetism
• Quantum phase transitions
• 2-dimensional systems
• Nuclear magnetic resonance