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

Abstract

Following the discovery of superconductivity near and above 50 K in oxypnictide compounds, the hunt is on for the mechanism of this unconventional superconductivity. The first measurements of the superconducting gap in the high temperature superconductor SmFeAsO0.85F0.15 give some clues. This is a crucial physical quantity, encoding qualitative and quantitative information about the nature of the superconducting state and the pairing interactions that lead to it. Chen et al. report the observation of a single gap in this superconductor. The size and detailed temperature dependence of the gap are close to that expected of a conventional 'BCS' single gap, s-wave superconductor, in contrast to the d-wave pseudogap features found in high-Tc cuprates and the two-gap structure observed in MgB2. The observation of a single gap in the superconductor SmFeAsO0.85F0.15 is reported. The gap value, its decrease with temperature and vanishing at Tc are all consistent with the Bardeen–Cooper–Schrieffer predictions, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. The results are not compatible with models involving antiferromagnetic fluctuations, strong correlations, t-J model, and the like, originally designed for the high-Tc copper oxides. Since the discovery of superconductivity in the high-transition-temperature (high-Tc) 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-xF x has recently been discovered1,2,3,4,5,6,7,8 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 SmFeAsO0.85F0.15 with Tc = 42 K as measured by Andreev spectroscopy. The gap value of 2Δ = 13.34 ± 0.3 meV gives 2Δ/kBTc = 3.68 (where kB is the Boltzmann constant), close to the Bardeen–Cooper–Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at Tc 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-Tc copper oxides.