A Sign-Changing Gap

Abstract

Physics Superconductors owe some of their fascinating properties to the formation of so-called Cooper pairs of electrons that (loosely) bind to each other thanks to a mediating interaction. The finite energy needed to break them apart is set by a function called “gap,” which, for unconventional superconductors, may have a complicated dependence on the momenta of the participating electrons. Much effort has been expended to elucidate the gap function in iron-based superconductors, many of which have multiband electronic structures that favor antiferromagnetism. A compatible gap function that changes sign going from an electron to a hole band was proposed and observed in some of those compounds, but the question remains whether this so-called symmetry is universal among the iron-based superconductors, and in particular whether it holds for the material LiFeAs, which lacks a static magnetic phase. Chi et al. use scanning tunneling and photoemission spectroscopy measurements coupled with calculations to analyze the scattering from impurities in LiFeAs. This scattering is sensitive to the change in sign of the gap function, leading to simple selection rules, and the authors find that their data support the gap scenario, and by implication, a magnetism-mediated superconductivity. Phys. Rev. B 89 , 104522 (2014).