The two dimensional Heisenberg model for SrCu 2 ͑BO 3 ͒ 2 has an exact dimer ground state which was proven by Shastry and Sutherland almost twenty years ago. The critical value of the quantum phase transition from the dimer state to the Néel ordered state is determined. Analysis of the experimental data shows that SrCu 2 ͑BO 3 ͒ 2 has the dimer ground state but its coupling constants are close to the transition point, which leads to an unusual temperature dependence of the susceptibility. The almost localized nature of the triplet excitations explains the plateaus observed in the magnetization curve.[S0031-9007(99)09090-0]

We report the observation of magnetic superstructure in a magnetization plateau state of SrCu 2 (BO 3 ) 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.Published in Science 298, 395 (2002).

We achieve a comprehensive understanding of the magnetic excitations observed by electric spin resonance and far-infrared spectroscopy in a frustrated spin gap system SrCu2(BO3)2 by considering the effects of magnetoelectric couplings and Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland model. The transitions from the dimer singlet ground state to the triplon and the bound states of two triplons are electroactive through the magnetoelectric couplings, even in the Shastry-Sutherland Heisenberg model. The results indicate that an electro-triplon and a novel electroactive magnetic excitation, as with an electromagnon in the multiferroics, can be realized in the conventional spin-gapped singlet and anomalous spin-liquid states. Clarifying these electroactive magnetic excitations in various spin systems will help analyze a broad range of quantum magnets, e.g., quantum spin liquids and spin nematics.