Pairing with Spin Fluctuations

Superconductivity requires a “pairing interaction”—an indirect attractive force between the conduction electrons that can overcome their direct Coulomb repulsion. To prove which particular degrees of freedom provide this pairing interaction is supremely difficult: nearly 40 years elapsed between the discovery of superconductivity in 1911 and the “smoking gun” measurement of the isotope effect that proved that lattice distortions provide the pairing interaction in conventional superconductors, and 26 years have elapsed since the discovery of high-temperature superconductivity, and still the pairing mechanism in cuprates is hotly debated. Thus it is a remarkable step forward that NMRmeasurements reported in Physical Review Letters by Taisuke Hattori of Kyoto University, Japan, and colleagues [1] have provided strong evidence that ferromagnetic fluctuations provide the pairing force that leads to superconductivity in an exotic, and comparatively recently discovered [2], class of superconductors in which ferromagnetism and superconductivity seem to happily coexist. Knowledge of the pairing mechanism will lead to a deeper understanding of the superconducting states of these, and possibly other, unconventional superconductors.