Static and dynamic spin correlations in pure and dopedLa2CuO4

We report elastic, quasielastic (F dE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin fluctuations in as-grown and doped ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. Four samples have been studied: (A) as-grown ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ with $T_N$=195 K, (B) oxygenated ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ with ${\mathit{T}}_{\mathit{N}}$≃100 K, (C) ${\mathrm{La}}_2$ ${\mathrm{Cu}}_{0.95}$ ${\mathrm{Li}}_{0.05}$ ${\mathrm{O}}_4$, and (D) ${\mathrm{La}}_{1.97}$ ${\mathrm{Sr}}_{0.03}$ ${\mathrm{Cu}}_{0.95}$ ${\mathrm{Li}}_{0.05}$ ${\mathrm{O}}_4$. All crystals exhibit variable-range-hopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetic instantaneous correlations in the ${\mathrm{CuO}}_2$ sheets with correlation length varying from ∼200 Å in crystal A to ∼14 Å in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and B the 2D correlation length becomes sufficiently large with decreasing temperature that the interplanar coupling is able to drive a transition to 3D long-range order. The spin dynamics have been studied in detail in crystals A and B and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no significant E≃0 component for temperatures ≥$T_N$ and instead the 2D response function is highly inelastic. The effective dispersion of the spin excitations is ≥0.4 eV Å. This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ in which the pairing is magnetic in origin.