Energy gap evolution across the superconductivity dome in single crystals of (Ba K )FeAs.

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba K )FeAs has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping ( ≈ 0.4) to becoming nodal at > 0.