| The frustrated 2D dimer spin system SrCu2(BO3)2 shows magnetization plateaus at 1/8, 1/4 and 1/3 of the saturated magnetization, where symmetry breaking superlattices of triplets were observed by NMR experiments (Science 298 (2002) 395). How the spin density distribution evolves between the plateaus is a highly non-trivial issue. One possibility is that the hopping of triplets gmeltsh the superlattice and the Bose-condensed phase with the transverse AF order appears instead. Another possibility is the "supersolid" phase, in which a superlattice of the longitudinal component coexists with the transverse AF order. The nature of the phase may be even more complicated by the presence of Dzyaloshinskii-Moriya interaction, which produces an effective staggered field (J. Phys.:Condens Matter. 17 (2005) L61). We performed 11B NMR experiments in the field up to 31 T above the 1/8-plateau phase. The NMR spectra show discontinuous change upon leaving the 1/8-plateau. However, the maximum hyperfine filed at the B nuclei remains nearly unchanged, indicating that the superlattice of largely polarized triplet dimers still persists above the 1/8 plateau phase. The ordering temperature in the high field phase agrees with the peak of the specific heat reported by Tsujii et al. ( cond-mat/0301509). The sharp fine structures of the NMR spectra in the plateau phase, however, become broadened as the field increases. Possible scenarios to explain the evolution of NMR spectra will be discussed. This work was done in collaboration with S. Matsubara1, M. Horvatic, C. Berthier, H. Kageyama and Y. Ueda. |
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