Vortex pinning: A probe for nanoscale disorder in iron-based superconductors

The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density j _c at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to weak collective pinning and a magnetic field-independent contribution j _c ^coll. The latter allows one to estimate quasiparticle scattering rates.