Point-contact Andreev-reflection spectroscopy in ReFeAsO1-xFx (Re = La, Sm): Possible evidence for two nodeless gaps

Abstract

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with T-c similar to 27 K and SmFeAsO0.8F0.2 (Sm-1111) polycrystals with T-c similar to 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB2. The single-band Blonder-Tinkham-Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO1-xFx (Re = La, Sm): a small gap, Delta(1), smaller than the BCS value (2 Delta(1)/kBT(c) similar to 2.2-3.2) and a much larger gap Delta(2) which gives a ratio 2 Delta(2)/kBT(c) similar to 6.5-9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T-c and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T-c. The normal-state conductance reproducibly shows an unusual, but different. shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T similar to 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound. (C) 2009 Elsevier B.V. All rights reserved.