Experimental study of the intrinsic multiple Andreev reflections effect in GdO(F)FeAs superconductor array junctions

We report the first observation of the intrinsic multiple Andreev reflections effect (IMARE) in S-n-S-...-S arrays (S = superconductor, n = normal metal) formed by a "break-junction" technique in the GdO(F)FeAs superconductor (TC = (48–53) K). We show that superconducting gap peculiarities in dI/dV spectra sharpen dramatically in the arrays as compared with that in the single-contact spectra; this enables to improve significantly the accuracy of the bulk superconducting parameters determination. By using IMARE, we determined the large- and the small-gap values ΔL = 11.0 ± 1.1 meV and ΔS = 2.6 ± 0.4 meV. The BCS ratio 2ΔL/kBTClocal = 5.0–5.9 > 3.52 (TClocal is the contact area critical temperature) evidences a strong electron-boson coupling. The results obtained agree well with our previous data by Andreev spectroscopy for single SnS contacts.