Flux distribution and penetration depth measured by muon spin rotation in high-T_{c} superconductors

The determination of the magnetic penetration depth λ in type-II superconductors from the field distribution n(B) measured by muon-spin rotation (μSR) is discussed. Particular stress is put on high-$T_c$ oxide superconductors (coherence length ξ≪λ; upper critical field much greater than the applied field; anisotropy and pinning). Fitting of the highly asymmetric n(B) by a Gaussian or Lorentzian, as done in existing μSR experiments, yields for a perfect vortex lattice λ values which are too large, and for a strongly distorted (due to pinning) lattice λ values which are too small. An improved evaluation of μSR data is suggested.