Neutron scattering study of the magnetic excitations in metallic and superconducting ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$

We report on neutron scattering studies of the dynamic spin correlations in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$. Two superconducting and one nonsuperconducting single crystals were studied. All three of the samples exhibited metallic normal-state resistivity modified by corrections consistent with localization effects. The structure factor S(Q,ω) measured in these samples reflects to a first approximation short-range two-dimensional antiferromagnetic spin correlations closely related to those found in insulating ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. In addition, S(Q,ω) measured in the superconducting samples has an incommensurate, double-peaked structure which is not present in the nonsuperconducting crystal. The integrated ( F dQ) intensities of the scattering at both concentrations are nearly independent of temperature over the range $k_B$T≫ħω to $k_B$T≪ħω for energy transfers greater than ∼6 meV. Furthermore, the difference in the temperature dependence of the intensity between the nonsuperconducting and superconducting samples for energy transfers less than ∼6 meV suggests that a gap may be forming in the magnetic excitation spectrum of the superconducting samples. The relationship of these results to the superconductivity in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ is discussed.