Spin Susceptibility in the Superconducting State of Cuprates

An analytic expression for the spin susceptibility in the superconducting state was derived taking into account strong correlation effects. Starting from a Hubbard model that also accounts for the repulsion between doped holes, the equations of motion were solved using a decoupling scheme. Since the commutator algebra of the model is different from that valid in the weak-coupling conventional Fermi liquid picture, the general expression for the susceptibility is quite different from the standard Pauli--Lindhard formula. We have then evaluated $\chi(q,\omega)$ using parameter values adopted to Fermi surfaces and neutron scattering data. The temperature dependence of the spin susceptibility is calculated for both s-- and d--wave pairing symmetry. The results are compared with NMR and NQR experiments on the spin--lattice and spin--spin relaxation rates as well as with features observed in neutron scattering experiments.