Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline SrCu 2 ͑BO 3 ͒ 2 having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system ͑D 30 K͒ in which the ground state can be solved "exactly." Furthermore, in the magnetization, two plateaus corresponding to 1 4 and 1 8 of the full Cu moment were first observed for 2D quantum spin systems. [S0031-9007(99)08878-X]

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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).

All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe 2 S 3 , a Mott insulator with striped-type magnetic ordering below ~120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below T c = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.The discovery of iron-based superconductors had a significant impact on condensed matter physics and led to extensive study of the interplay between crystal structure, magnetism and superconductivity 1 . All the iron-based superconducting materials discovered to date share the same structural motif: a two-dimensional square lattice formed by edge-shared FeX 4 tetrahedra (X = Se, P and As). The Fe atoms are nominally divalent in most of the parent materials. These parent compounds undergo a magnetic transition at low temperatures, typically exhibiting striped-type ordering.Superconductivity appears when the magnetic order is fully suppressed by the application of pressure or by the addition of doping carriers through chemical Purpose of this studyThe application of pressure is often a useful means of changing the electronic structure of a compound so as to induce a metallic state without simultaneously introducing any degree of disorder 17 . In this study, we investigated in detail the magnetic properties of a sulphur-analogue of the Fe-based ladder materials, BaFe 2 S 3 (space group: orthorhombic, Cmcm) 18,19 , and undertook experimental trials in which this compound was subjected to high pressures to obtain the metallic state. The electronic properties of this material depend on the manner in which the samples are synthesized, and thus we present data for sample 1 describing magnetic properties, and data for a range of samples 1 to 6 describing high-pressure effects. The details of the sample preparation process are given in the Method section. Electronic properties under ambient pressureFigure 2a displays the temperature dependence of the electrical resistivity (ρ) of BaFe 2 S 3 along the leg direction under ambient pressure. The observed insulating behaviour, which occurs despite the expected metallic behaviour in an unfilled 3d manifold, is caused by the Coulomb repulsion between Fe 3d electrons, which becomes prominent in a quasi-one-dimensional ladder structure. Figure 2b shows the magnetic susceptibility (χ) at 5 T along the three orthorhombic...

High-field magnetization measurements up to 57 T have been performed at 0.08 K in a single crystal of a two-dimensional spin-gap material SrCu 2 (BO 3 ) 2 . We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic superstructure is characterized by a novel stripe order of triplets. The 1/3 plateau is much wider than the previously observed 1/4 and 1/8 plateaux.

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Coherent spin oscillations were nonthermally induced by circularly polarized pulses in the fully compensated antiferromagnet NiO. This effect is attributed to the action of the effective magnetic field generated by an inverse Faraday effect on the spins. The novelty of this mechanism is that spin oscillations are driven by the time derivative of the effective magnetic field which acts even on "pure" antiferromagnets with zero net magnetic moment in the ground state. The measured frequencies (1.07 THz and 140 GHz) correspond to the out-of-plane and in-plane modes of antiferromagnetic spin oscillations.

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