Abstract
Point-contact Andreev reflection spectroscopy is applied to investigatethe gap structure in iron pnictide single-crystal superconductors of theAFe2As2 (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 andBa(Fe0.9Co0.1)2As2, and theother with a V2/3 background conductance universally observed, extending even up to 100 meV forSr0.6Na0.4Fe2As2 andSr(Fe0.9Co0.1)2As2.The latter is also observed in point-contact junctions on the nonsuperconducting parent compoundBaFe2As2 andsuperconducting (Ba0.6K0.4)Fe2As2 crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders isconsidered to explain distinct behaviors in the superconducting samples. ForBa0.6K0.4Fe2As2, double peaks due to Andreev reflection with a strongly slopingbackground are frequently observed for point contacts on freshly cleavedc-axis surfaces. If normalized using a background baseline and analyzedusing the Blonder–Tinkham–Klapwijk model, the data show a gap size of ∼ 3.0–4.0 meVwith 2Δ0/kBTc ∼ 2.0–2.6, consistent with the smaller gap size reported for the LnFeAsO family (‘Fe-1111’). For theBa(Fe0.9Co0.1)2As2, theG(V) curves typically display a zero-bias conductance peak.
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