Charge doping of iron-pnictide superconductors leads to collective pinning of
flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the
charge-doped compounds is consistently described by the mean-free path
fluctuations introduced by the dopant atoms, allowing for the extraction of the
elastic quasiparticle scattering rate. The absence of scattering by dopant
atoms in isovalently doped BaFe2(As1−xPx)2 is consistent
with the observation of a linear temperature dependence of the low-temperature
penetration depth in this material.Comment: 4 page