A magnetic susceptibility which decreases with decreasing temperature is observed in all Cu${\mathrm{O}}_2$ based superconductors with less than optimal doping. We propose that in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ this is due to spin density wave ordering which is prevented by the low spatial dimensionality, while in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ it is due to the interplay between antiferromagnetic fluctuations within a plane and singlet pairing of electrons between nearest neighbor planes.

• Received 14 August 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2810