1. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
   
2. Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

Correspondence to: C. L. Chien¹ Correspondence and requests for materials should be addressed to C.L.C. (Email: clc@pha.jhu.edu).

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^{1, 2, 3, 4, 5, 6, 7, 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 SmFeAsO_0.85F_0.15 with T _c = 42 K as measured by Andreev spectroscopy. The gap value of 2 = 13.34  0.3 meV gives 2/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.