Flux distribution and penetration depth measured by muon spin rotation in high-Tc superconductors

The determination of the magnetic penetration depth {λ} in type-II superconductors from the field distribution n(B) measured by muon-spin rotation ({μ}SR) is discussed. Particular stress is put on high-Tc 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 {μ}SR 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 {μ}SR data is suggested.