Dependence of the absolute value of the penetration depth in $(\mathbf{B}{\mathbf{a}}_{1-\mathbf{x}}{\mathbf{K}}_{\mathbf{x}})\mathbf{F}{\mathbf{e}}_2\mathbf{A}{\mathbf{s}}_2$ on doping

Abstract

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor $\mathrm{B}{\mathrm{a}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth (${\lambda }_{ab}$) in underdoped, optimally-doped, and overdoped samples. The results suggest an abrupt increase of ${\lambda }_{ab}$ as doping is increased from $x_{\mathrm{opt}}$, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for $x=0.19$ and $0.58$ is compatible with previously observed structural symmetries at those doping levels.

Replaced Fig. 5 and 6, minor changes to the rest of the text