Spin waves and superconductors

Ever since the discovery of high-temperature superconductivity in a family of copper oxides, or cuprates, theorists and experimentalists have struggled to understand the mechanism behind the phenomenon. Is it something akin to conventional superconductivity, in which a weak attraction between fermionic electrons pairs them up into bosonic bound states? Or might some fundamentally new theory be required? In a conventional superconductor, explained by the Bardeen-Cooper-Schrieffer (BCS) theory, electrons are held together in pairs by an attraction mediated by lattice phonons. The BCS theory also works for pairing mediated by other lattice excitations, but until recently, phonons were the only candidate known to be present in all the superconducting cuprates, and a phonon-mediated interaction didn’t seem to do the trick. Now, researchers led by Bernhard Keimer (Max Planck Institute for Solid State Research) have produced compelling evidence in favor of a different possibility: electron pairing mediated by short-range spin waves known as paramagnons. It’s been known, theoretically, that sufficiently energetic paramagnons can produce the necessary attractions between electrons, but in all but a few of the superconducting cuprates, only weak paramagnons had ever been seen. Keimer and colleagues used a recently developed technique, called resonant inelastic x-ray scattering, to study a wide range of superconducting cuprates, and found strong paramagnons in all of them. Although the best available model of magnetically induced superconductivity is still somewhat crude, the researchers were able to use it, together with their observed paramagnon spectra, to reproduce the cuprates’ critical temperatures within a factor of two; the challenge now is for theorists to do better. (M. Le Tacon et al., Nat. Phys., in press, doi:10.1038/nphys2041 .) —Johanna Miller