Point-contact spectroscopic studies on normal and superconducting AFe ₂As₂-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 ₂As₂ (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 (Ba _0.6K_0.4)Fe₂As₂ and Ba(Fe _0.9Co_0.1)₂As₂, and the other with a V^2/3 background conductance universally observed, extending even up to 100meV for Sr_0.6Na_0.4Fe₂As₂ and Sr(Fe_0.9Co_0.1)₂As₂. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe₂As₂ and superconducting (Ba _0.6K_0.4)Fe₂As₂ crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba _0.6K_0.4Fe₂As₂, 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 ∼ 3.0-4.0meV with 2Δ₀/k _BT_c ∼ 2.0-2.6, consistent with the smaller gap size reported for the LnFeAsO family ('Fe-1111'). For the Ba(Fe_0.9Co _0.1)₂As₂, the G(V) curves typically display a zero-bias conductance peak.