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apr11-offprint-taillefer.pdf

In several families of materials, including the recently discovered iron pnictides, super-conductivity is found near the quantum critical point where a magnetic phase ends, pointing to a magnetic glue as the source of electron pairing, distinct from the usual phononic glue. In the copper oxides where superconductivity persists to the highest temperatures, the nature and role of an elusive quantum critical point are the subject of much debate. Is the glue again magnetic? If so, why is it so strong in this case?