Spin gaps and spin dynamics in La2-xSrxCuO4 and YBa2Cu3O7- delta.

A magnetic susceptibility which decreases with decreasing temperature is observed in all CuO 2 based superconductors with less than optimal doping. We propose that in La 2-x Sr x CuO 4 this is due to spin density wave ordering which is prevented by the low spatial dimensionality, while in YBa 2 Cu 3 O 6.6 it is due to the interplay between antiferromagnetic fluctuations within a plane and singlet pairing of electrons between nearest neighbor planes