An elastic neutron scattering study has been performed on several single crystals of La2−xSrxCuO4 for x near the lower critical concentration xc for superconductivity. In the insulating spin-glass phase (x = 0.04 and 0.053), the previously reported one-dimensional spin modulation along the orthorhombic b-axis is confirmed. Just inside the superconducting phase (x = 0.06), however, two pairs of incommensurate magnetic peaks are additionally observed corresponding to the spin modulation parallel to the tetragonal axes. These two types of spin modulations with similar incommensurabilities coexist near the boundary. The peak-width κ along spin-modulation direction exhibits an anomalous maximum in the superconducting phase near xc, where the incommensurability δ monotonically increases upon doping across the phase boundary. These results are discussed in connection with the doping-induced superconducting phase transition.

Neutron scattering experiments continue to improve our knowledge of spin fluctuations in layered cuprates, excitations that are symptomatic of the electronic correlations underlying high-temperature superconductivity. Time-of-flight spectrometers, together with new and varied single crystal samples, have provided a more complete characterization of the magnetic energy spectrum and its variation with carrier concentration. While the spin excitations appear anomalous in comparison with simple model systems, there is clear consistency among a variety of cuprate families. Focusing initially on hole-doped systems, we review the nature of the magnetic spectrum, and variations in magnetic spectral weight with doping. We consider connections with the phenomena of charge and spin stripe order, and the potential generality of such correlations as suggested by studies of magnetic-field and impurity induced order. We contrast the behavior of the hole-doped systems with the trends found in the electron-doped superconductors. Returning to hole-doped cuprates, studies of translation-symmetry-preserving magnetic order are discussed, along with efforts to explore new systems. We conclude with a discussion of future challenges.

Neutron scattering experiments reveal that a diagonal spin modulation, which is a one-dimensional modulation rotated away by 45 • from that in the superconducting phase, occurs universally across the insulating spin-glass phase in La2−xSrxCuO4 (0.02≤ x ≤0.055). This establishes an intimate relation between the magnetism and the transport properties in the high-temperature copper oxide superconductors. Furthermore, it is found that the charge density per unit length estimated using a charge stripe model is almost constant throughout the phase diagram, even when the modulation rotates away by 45 • at the superconducting boundary. However, at the lowest values for x the density changes approaching 1 hole/Cu as in La2−xSrxNiO4. Magnetic excitation spectra suggest that magnetic correlations change from incommensurate to commensurate at ω ∼7 meV and T ∼70 K, indicating a characteristic energy for the incommensurate structure of 6-7 meV.

High-temperature (high-T c ) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state 1,2 . On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates 1 . Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides 3-6 . Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped

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