A BCS-like gap in the superconductor SmFeAsO0.85F0.15

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 LaFeAsO1-xFx has recently been discovered(1-8) 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 SmFeAsO0.85F0.15 with T-c= 42 K as measured by Andreev spectroscopy. The gap value of 2 Delta = 13.34 +/- 0.3meV gives 2 Delta/k(B)T(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.