Specific heat of the S=1/2 two-dimensional Shastry-Sutherland antiferromagnet SrCu2(BO3)2 in high magnetic fields

We characterize the field-induced magnetic phases of SrCu2(BO3)2, a frustrated spin-1/2 Heisenberg antiferromagnet in the two-dimensional Shastry-Sutherland lattice, using specific heat in magnetic fields up to 33 T. We find that the spin gap persists above the expected critical field Hc = Δ/gμB of 21 T despite the appearance of magnetic moment in the ground state. At the magnetization plateau at 1/8 of the saturation, the triplets that carry the magnetization of the ground state are observed to form a two-dimensional gas of large mass. A gas consisting of the same number of heavy particles continues to completely dominate the specific heat in the field region above the plateau, although the magnetization increases with increasing field. Ordering is observed immediately below the temperature region of the gas phase. (author)