The London penetration depth in BaFe₂As₂ superconductors at high electron doping level^*

We compare the temperature dependences of the in-plane London penetration depth (Δλ_ab(T)) for several Ba(Fe_{1 − x}TM_x)₂As₂ (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 , 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 Δλ_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.