Ni-Impurity Effects on Incommensurate Spin Correlations in the Superconducting Phase of La_<2_x>Sr_xCuO_4 near the Spin-Glass Phase Boundary(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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Neutron scattering experiments were carried out to explore Ni-impurity effects on static spin correlations in La_<2_x>Sr_xCuO_4 in the vicinity of the superconductor-insulator boundary where both parallel and diagonal spin-density modulations (SDMs) coexist at low temperature. Dilute Ni substitution completely destroys the bulk superconductivity and alters well-defined incommensurate peaks of Ni-free SDMs into a single broad peak at (π,π). As the Ni concentration (y) reaches 〜x, the magnetic diffuse scattering is transformed to an intense sharp Bragg peak, corresponding to a three-dimensional antiferromagnetic (AF) order in bulk. T_N of such Ni-induced AF order depends on x and appears to disappear at x 〜 0.1. A simplified cross section for the Q spectra taking into account the orthorhombic domain structure reproduces the Ni-impurity effect on the static spin correlations well, which results in a decreasing incommensurability of both types of SDMs. These effects are approximately ascribed by a reduction in the number of mobile holes, or by the effective hole number x_<eff>=x-y, similarly to the case of diagonal SDMs in insulating spin-glasses.
社団法人日本物理学会の論文
2007-07-15