Finite gap behaviour in the superconductivity of the 'infinite layer' n-doped high-T-c superconductor Sr0.9La0.1CuO2

We report on the first small-angle neutron scattering measurements from the flux line lattice (FLL) in the high-T-c cuprate superconductor Sr0.9La0.1CuO2. Using a polycrystalline sample, the scattered intensity decreases monotonically with scattering angle away from the undiffracted beam, independently of the azimuthal angle around the beam. The absence of clear peaks in the intensity suggests the establishment of a highly disordered FLL within the grains. We find that the intensity distribution may be represented by the form factor for a single flux line in the London approximation, with some contribution from crystal anisotropy. Most interestingly however, we find that, over the observed field range, the temperature dependence of the diffracted intensity is best represented by s-wave pairing, with lower limits of the gap values being very similar to the Bardeen-Cooper-Schrieffer value of Delta(0) = 1.76 k(B)T(c). However, a qualitative consideration of corrections to the observed intensity suggests that these gap values are likely to be higher, implying strong-coupling behaviour.