Zero-field superfluid density in d-wave superconductor evaluated from the results of muon-spin-rotation experiments in the mixed state

We report on measurements of the in-plane magnetic penetration \lambda_{ab} in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+\delta (OP Bi2201) by means of muon-spin rotation (\muSR). We show that in unconventional $d-$wave superconductors (like OP Bi2201), \muSR experiments conducted in various magnetic fields allow to evaluate the zero-field magnetic penetration depth \lambda_0, which relates to the zero-field superfluid density in terms of \rho_s\propto\lambda_0^-2.