Octet-Line Node Structure of Superconducting Order Parameter in KFe 2 As 2

Abstract

An Eight-Noded Monster In superconductors, electrons are bound into pairs, and the exact form of that pairing and the resulting energy gap can vary, depending on the details of the electron-electron interaction and the band structure of the material. The energy gaps of the recently discovered iron-based superconductors exhibit a variety of pairing functions. KFe 2 As 2 has been suggested to have a d -wave gap, similar to cuprate superconductors. Okazaki et al. (p. 1314 ) use laser-based angle-resolved photoemission spectroscopy (ARPES) to map out the superconducting gap on three Fermi surfaces (FS) of the compound. They find a different gap structure on each, with the middle FS gap vanishing at eight distinct positions (nodes). It appears that the gap respects the tetragonal symmetry of the crystal, indicating (although the details may vary) the all iron-based superconductors have an extended s -wave–symmetric pairing—a finding that will help understanding of unconventional superconductivity.