Description
We have used magnetic force microscopy (MFM) to
characterize superconductivity locally across the
superconducting dome in the electron, hole, and isovalently
doped iron pnictides [$Ba_{1-x}K_xFe_2As_2, Ba(Fe_{1-
x}Co_x)_2As_2, BaFe_2(As_{1-x}P_x)_2$]. We find a sharp
increase of the absolute value of the penetration depth (?ab)
upon approaching the underdoped edge of the
superconducting dome, which suggests the superfluid density
$n_s=1/\lambda_{ab}^2$ vanishes. On the overdoped side
$\lambda_{ab}$ flattens out. The behavior around optimal
doping depends on the material. In $Ba(Fe_{1-
x}Co_x)_2As_2$ we observe a weak minimum of
$\lambda_{ab}$. In $BaFe_2(As_{1-x}P_x)_2$ we find a
clear maximum and in $Ba_{1-x}K_xFe_2As_2$ what
appears to be an abrupt jump in $\lambda_{ab}$. This hints
at the possible existence of a phase that gives rise to mass
renormalization. In all samples except underdoped ones
superconductivity is homogenous, even around optimal
doping. Very underdoped samples are less homogeneous
because of local variations of doping. In mildly underdoped
$BaFe_2(As_{1-x}P_x)_2$ we observe stripes of enhanced
diamagnetic response.
