Measurements of the superconducting gap in a new member of the family of iron-based superconductors revealed substantial deviation from predictions of the well-established theory. In superconductors, electrons form Cooper pairs that behave like single particles. The force binding the electrons in pairs, known as the superconducting gap, often depends on the momentum of the electrons and its measurement shows fingerprints of the mechanism that causes superconductivity. Researchers used a technique known as laser angle-resolved photoemission spectroscopy combined with electronic structure calculations to measure this quantity in a new calcium-potassium-iron arsenide superconductor. Surprisingly, they found that the variation of the superconducting gap with momentum strongly deviates from expectations. This work provides new insights for constructing a global model of high temperature superconductivity in iron-based materials.
Loci of the measured momentum dependence of the superconductor gap of the Fermi surface sheets. The dashed line shows the expected variation of the gap based on a spin fluctuation model. The significant deviation from predictions demonstrates the deficiency of current models.
Enhancement of the Superconducting Gap by Nesting in CaKFe4As4: A New High Temperature Superconductor