Studies of New Superconductors Revive Old Questions

Lanthanum copper oxide, which is now regarded as the prototype for the new high‐temperature superconductors, undergoes a phase transition to an antiferromagnetic state. The critical temperature for this transition, at which the magnetic moments on copper ions begin to order antiferromagnetically, depends sensitively on the oxygen concentration. Confirmation of the existence of this antiferromagnetic phase in ${\mathrm{La}}_2{\mathrm{CuO}}_{\text{4−y}}$ and the determination, using neutron scattering, of the arrangement of magnetic moments in it provided definitive evidence last summer for the importance of magnetic phenomena in the new superconducting oxides. Philip Anderson (Princeton University) had proposed in January 1987 that superconductivity in ${\mathrm{La}}_2{\mathrm{CuO}}_{\text{4−y}}$ doped with barium or strontium, which had been confirmed only two months earlier, arose from novel, short‐range antiferromagnetic correlations between copper spins.