Studying electronic excitations by intentionally creating point-like defects in the crystal lattice has helped distinguish between competing electron pairing states and led to a better understanding of the origins of superconductivity in barium-potassium-iron-arsenide. By adjusting two independent "knobs", the ratio of barium and potassium and the scattering by defects introduced by 2.5 MeV electron irradiation, the response of two key independent parameters, superconducting transition temperature, Tc, and low-temperature magnetic susceptibility is compared to theoretical predictions. Measuring samples with different ratios of barium to potassium, each with variable induced defect densities, reveals a sharp break in the London penetration depth and the rate of Tc suppression. If spin fluctuations are responsible for superconductivity, they should result in a unique sign-changing pairing state, called s±, where the sign of the superconducting order parameter differs on opposing Fermi surfaces. Such a pairing state is prone to non-magnetic disorder, which is used to distinguish it from conventional, s-wave, pairing. The results imply that s± pairing is very robust across the composition range and survives an abrupt transition from a nodeless to a nodal superconducting gap with increasing potassium content.
Suppression of the superconducting transition by controlled disorder for different compositions.
Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (Ba1‑xKx)Fe2As2