Dependence of the absolute value of the penetration depth in $\mathrm{\mathbf{{(Ba_{1-x}K_x)Fe_2As_2}}}$ on doping

TL;DR

This study uses magnetic force microscopy to measure the in-plane magnetic penetration depth in Ba₁₋ₓKₓFe₂As₂ across different doping levels, revealing a potential quantum critical point and structural symmetry effects.

Contribution

It provides the first local measurements of the absolute penetration depth in this superconductor across various doping levels, highlighting abrupt changes near optimal doping.

Findings

Abrupt increase of λ_ab near optimal doping

Compatibility of vortex response with structural symmetries

Potential link to quantum critical point

Abstract

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor $\mathrm{Ba_{1-x}K_xFe_2As_2}$. 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.