Point-contact spectroscopic studies on normal and superconducting AFe2As2-type iron pnictide single crystals

Point-contact Andreev reflection spectroscopy is applied to investigate the gap structure in iron pnictide single-crystal superconductors of the AFe(2)As(2) (A = Ba, Sr) family ('Fe-122'). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba0.6K0.4) Fe2As2 and Ba(Fe0.9Co0.1)(2)As-2, and the other with a V-2/3 background conductance universally observed, extending even up to 100 meV for Sr0.6Na0.4Fe2As2 and Sr(Fe0.9Co0.1)(2)As-2. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe2As2 and superconducting (Ba0.6K0.4)Fe2As2 crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba0.6K0.4Fe2As2, double peaks due to Andreev reflection with a strongly sloping background are frequently observed for point contacts on freshly cleaved c-axis surfaces. If normalized using a background baseline and analyzed using the Blonder-Tinkham-Klapwijk model, the data show a gap size of similar to 3.0-4.0 meV with 2 Delta(0)/k(B)T(c)similar to 2.0-2.6, consistent with the smaller gap size reported for the LnFeAsO family ('Fe-1111'). For the Ba(Fe0.9Co0.1)(2)As-2, the G(V) curves typically display a zero-bias conductance peak.