A BCS-like gap in the superconductor SmFeAsO_0.85F_0.15

Since the discovery of superconductivity in the high-transition-temperature (high-T_c) copper oxides two decades ago, it has been firmly established that the CuO₂ plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO_1-xF _x has recently been discovered and the conspicuous lack of the CuO₂ planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO _0.85F_0.15 with T_c = 42 K as measured by Andreev spectroscopy. The gap value of 2Δ = 13.34 ± 0.3 meV gives 2Δ/k_BT_c = 3.68 (where k_B is the Boltzmann constant), close to the Bardeen-Cooper-Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at T_c in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-T_c copper oxides.