Evidence of a universal and isotropic 2\Delta/kBTC ratio in 122-type iron pnictide superconductors over a wide doping range

摘要

We have systematically investigated the doping and the directional dependenceof the gap structure in the 122-type iron pnictide superconductors by pointcontact Andreev reflection spectroscopy. The studies were performed on singlecrystals of Ba1-xKxFe2As2 (x = 0.29, 0.49, and 0.77) and SrFe1.74Co0.26As2 witha sharp tip of Pb or Au pressed along the c-axis or the ab-plane direction. Theconductance spectra obtained on highly transparent contacts clearly showevidence of a robust superconducting gap. The normalized curves can be welldescribed by the Blonder-Tinkham-Klapwijk model with a lifetime broadening. Thedetermined gap value scales very well with the transition temperature, givingthe 2{\Delta}/kBTC value of ~ 3.1. The results suggest the presence of auniversal coupling behavior in this class of iron pnictides over a broad dopingrange and independent of the sign of the doping. Moreover, conductance spectraobtained on c-axis junctions and ab-plane junctions indicate that the observedgap is isotropic in these superconductors.