Antiferromagnetic spin correlations in insulating, metallic, and superconductingLa2−xSrxCuO4

Abstract

We have carried out elastic, quasielastic ($\int \mathrm{dE}$), and inelastic neutron scattering studies of the antiferromagnetic spin correlations in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ with $x$ varying between 0.02 and 0.18. The crystals, which were grown in three different laboratories, exhibit behavior that varies smoothly with $x$. In all cases, antiferromagnetic correlations with a scattering amplitude corresponding to a fully occupied ${\mathrm{Cu}}^{2+}$ square lattice are observed. However, the Néel state is destroyed by the doping and the spin-spin correlation length $\xi$ is quite short, varying from ∼35 to ∼8 Å as $x$ varies between 0.02 and 0.18; the local order is, however, the same as in pure ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. The fluctuations are dynamic in character as in ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ above the Néel temperature $T_N$. To a first approximation, $\xi =\frac{3.8}{\sqrt{x}}$ Å, the average separation between the holes introduced by the ${\mathrm{Sr}}^{2+}$ doping. The $x=0.08\mathrm{}$ sample exhibits superconductivity with $T_c=10\mathrm{}$ K and with a Meissner fraction exceeding 15% at 5 K; no important differences in the magnetic scattering are observed in the normal and superconducting states. In an appendix we present additional data on the spin dynamics in pure ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ at $T_c=300\mathrm{}$ K in a sample with $T_N=235\mathrm{}$ K.