Coexistence of two order parameters and a pseudogap in the iron-based superconductors

The number, the symmetry and the amplitude of the order parameters (OPs) in the Fe-As superconductors are still open issues, as well as the origin of the electron pairing. To address these issues, we performed point-contact Andreev- reflection measurements in SmFeAsO$_{0.8}$F$_{0.2}$ ($T_c^{on}=53 $ K) and LaFeAsO$_{0.9}$F$_{0.1}$ ($T_c^{on}=27$ K) polycrystals. In both cases, the low-temperature conductance curves clearly indicate the presence of two OPs in the superconducting state. No zero-bias peaks were observed, which -- considering the non-directional current injection -- clearly rules out the $d$-wave symmetry. If a superconducting character is supposed for both the OPs, their amplitudes can be extracted from a generalized two-band BTK fit (with two s-wave gaps, as in MgB$_2$) of the normalized conductance curves. The fit is indeed very good and gives OP amplitudes, $\Delta_1$ and $\Delta_2$, that lie slightly below and well above the BCS value, respectively. In Sm-1111, their low-temperature values are $\Delta_1(0)=6.15\pm0.50 $ meV and $\Delta_2(0)=18\pm3$ meV, which give gap ratios ($2 \Delta/k_BT_c$) of about 2.7 and 8.0. Both $\Delta_1$ and $\Delta_2$ show a BCS-like temperature dependence and close at the bulk $T_c$. In La-1111 we obtained point contacts with different local $T_c$ (from 27.3 to 31.0 K) in crystallites with slightly different doping. Here $\Delta_1$ shows a non-BCS temperature dependence with a high- temperature ``tail,'' while $\Delta_2$ seems to close at T<Tc.