We compare the temperature dependences of the in-plane London penetration depth ( λ ab (T )) for several Ba(Fe 1−x TM x ) 2 As 2 (TM = Co, Ni, Pd, Co + Cu) superconductors at high concentration of electrons, n e , added per Fe site. We show that regardless of the transition metal TM, for n e 0.12, the penetration depth has a power-law temperature dependence λ L (T ) ∝ T n , but with the exponent n ≈ 1.65, thus significantly lower than the value n 2 previously reported for lower electron concentrations. On doping with electrons from the 3d shells, the magnitude of the variation with temperature of λ L (T ) is larger for Ni substitution than for Co substitution, and larger for Co + Cu co-doping than for Ni co-doping. However, comparing the effect of 3d and 4d electrons, for the isovalent elements Ni and Pd respectively, we found that the rates of change in penetration depth with temperature are nearly identical for the two compounds.(Some figures in this article are in colour only in the electronic version) * This article was originally submitted for the special section 'Focus on the electromagnetic properties of iron-based superconductors', Superconductor Science and Technology, volume 23, issue 5.
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