Static and dynamic spin correlations in the spin-glass phase of slightly doped La_2-xSr_xCuO₄

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 La_2-xSr_xCuO₄ (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 La_2-xSr_xNiO₄. 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.