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.
@article{arxiv.cond-mat/0210294,
title = {Magnetic Superstructure in the Two-Dimensional Quantum Antiferromagnet SrCu2(BO3)2},
author = {K. Kodama and M. Takigawa and M. Horvatic and C. Berthier and H. Kageyama and Y. Ueda and S. Miyahara and F. Becca and F. Mila},
journal= {arXiv preprint arXiv:cond-mat/0210294},
year = {2009}
}