Magnetic Superstructure in the Two-Dimensional Quantum Antiferromagnet   SrCu2(BO3)2
  We report the observation of magnetic superstructure in a magnetization plateau state of SrCu2(BO3)2, a frustrated quasi-two-dimensional quantum spin system. The Cu and B nuclear magnetic resonance (NMR) spectra at 35 mllikelvin indicate an apparently discontinuous phase transition from uniform magnetization to a modulated superstructure near 27 tesla, above which a magnetization plateau at 1/8 of the full saturation has been observed. Comparison of the Cu NMR spectrum and the theoretical analysis of a Heisenberg spin model demonstrates the crystallization of itinerant triplets in the plateau phase within a large rhomboid unit cell (16 spins per layer) showing oscillations of the spin polarization. Thus we are now in possession of an interesting model system to study a localization transition of strongly interacting quantum particles. 
K. Kodama (1), M. Takigawa (1), M. Horvatic (2), C. Berthier (2,3), H.   Kageyama (1), Y. Ueda (1), S. Miyahara (1,4), F. Be...