On the interpretation of muon-spin-rotation experiments in the mixed state of type-II superconductors

We argue that claims about magnetic field dependence of the magnetic field penetration depth λ, which were made on the basis of muon-spin-rotation (μSR) studies of some superconductors, originate from insufficient accuracy of theoretical models employed for the data analysis. We also reanalyse some of already published experimental data and demonstrate that numerical calculations of Brandt [E.H. Brandt, Phys. Rev. B 68 (2003) 54506] may serve as a reliable and powerful tool for the analysis of μSR data collected in experiments with conventional superconductors. Furthermore, one can use this approach in order to distinguish between conventional and unconventional superconductors. It is unfortunate that these calculations have practically never been employed for the analysis of μSR data.