LOW-FREQUENCY SPIN FLUCTUATIONS IN THE SUPERCONDUCTING LA2-XSRXCUO4

Recent progress in low-energy neutron scattering study on the spatiall y modulated or so-called incommensurate spin fluctuations is reviewed. Well-defined incommensurate spin fluctuations are observed beyond x a pproximate to 0.05 and up to the highest Sr-concentration x = 0.25 in the present study. The incommensurability delta(x) is saturated at aro und 1/8 in the overdoped region and Linearly scaled with the upper Lim it of T-c(x) between the underdoped and optimally doped region. A char acteristic energy E- below which the dynamical magnetic susceptibilit y dramatically diminishes in the superconducting state can be determin ed to be around 7 meV in the energy spectrum of the spin fluctuations for x = 0.15 and x = 0.18 crystals. The disorder effect induces a low- frequency component of the incommensurate spin fluctuations below E-.