Superfluid Density and Angular Dependence of the Energy Gap in Optimally Doped (BiPb)₂(SrLa)₂CuO_6+δ

We present a muon-spin rotation study of the optimally doped cuprate superconductor (BiPb)2(SrLa)2 CuO6+δ. The measured magnetic field dependence of the in-plane magnetic penetration λab suggests superconductivity with a dominant d-wave order parameter. The comparison of the temperature dependence of λab with calculations, assuming the angular gap symmetry as obtained from photoemission measurements, is consistent with a partial suppression of the quasi-particle weight towards the anti-nodal region of the Fermi surface. This suggests that the superconducting and the pseudogap state are dominated by different parts of the Fermi surface.