CHARGE AND SPIN SUBSTITUTIONAL EFFECTS IN SUPERCONDUCTORS AND IN THEIR PRECURSOR ANTIFERROMAGNETS

A simple, largely qualitative but comprehensive description of the spi n-dynamics of Cu2+, as derived from NMR-NQR relaxation and shifts meas urements, is given for two-dimensional S = 1/2 Heisenberg antiferromag nets and for high-T(c) superconductors with charge and spin substituti ons. The insights that can be obtained are illustrated with reference to specific examples, including La2CuO4 With substitution of Cu2+ S = 1/2 by Ni2+ S = 1 and With Sr for La substitution leading to mobile va cancies. In the metallic phase it is shown how the transition to the s uperconducting state affects the spin dynamics by opening a pseudogap in the spin excitations. Then the effects of atomic substitutions caus ing variations in T(c) are addressed. The main conclusions on the spin dynamics and the possible role of the low-energy magnetic excitations in the mechanisms of high-T(c) superconductivity are outlined.