title: Magnetic superstructure in the two-dimensional quantum antiferromagnet SrCu2(BO3)2.
abstract: 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 millikelvin 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.
authors.affiliation: Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Kodama
authors.forename: M
authors.initials: M
authors.lastname: Takigawa
authors.forename: M
authors.initials: M
authors.lastname: Horvatić
authors.forename: C
authors.initials: C
authors.lastname: Berthier
authors.forename: H
authors.initials: H
authors.lastname: Kageyama
authors.forename: Y
authors.initials: Y
authors.lastname: Ueda
authors.forename: S
authors.initials: S
authors.lastname: Miyahara
authors.forename: F
authors.initials: F
authors.lastname: Becca
authors.forename: F
authors.initials: F
authors.lastname: Mila