Field dependence of the quantum ground state in the Shastry-Sutherland system SrCu ₂ (BO ₃ ) ₂

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We present magnetic torque measurements on the Shastry-Sutherland quantum spin system SrCu 2 (BO 3 ) 2 in fields up to 31 T and temperatures down to 50 mK. A new quantum phase is observed in a 1 T field range above the 1/8 plateau, in agreement with recent NMR results. Since the presence of the DM coupling precludes the existence of a true Bose-Einstein condensation and the formation of a supersolid phase in SrCu 2 (BO 3 ) 2 , the exact nature of the new phase in the vicinity of the plateau remains to be explained. Comparison between magnetization and torque data reveals a huge contribution of the Dzyaloshinskii-Moriya interaction to the torque response. Finally, our measurements demonstrate the existence of a supercooling due to adiabatic magnetocaloric effects in pulsed field experiments.