Researchers at Cambridge University in the UK have discovered that the electron pairs in certain superconductors are held together by "magnetic glue". Superconductivity is the complete loss of resistance to electrical current below a certain temperature. In conventional low-temperature superconductors, the electrons pairs that carry the supercurrent are bound together by vibrations of the crystal lattice known as phonons. However, Gilbert Lonzarich and colleagues at Cambridge have shown that under certain conditions the electrons appear to be held together by magnetic spin-spin interactions (N D Mathur et al. 1998 Nature 394 39).
The Cambridge team studied two heavy fermion compounds: CeIn3 and CePd2Si2. Heavy fermion compounds are materials in which the conduction electrons acquire masses hundreds of times those of free electrons as a result of interactions with magnetic moments within the material. The compounds exhibit many unusual properties, including superconductivity. Under most circumstances phonons are responsible for the superconductivity in heavy fermion systems. However, the Cambridge group found evidence for magnetically mediated superconductivity in extremely pure samples. For both materials the team measured the variation of the magnetic and superconducting transition temperatures with lattice density, and the variation of the resistivity of the normal (i.e. non-superconducting) state with temperature. The magnetic coupling was only observed at lattice densities lose to the critical density at which long-range magnetic order is suppressed.
The results could shed new light on the superconducting and normal-state properties of other strongly correlated electron systems including, perhaps, the high-temperature copper oxide superconductors.
