Local measurement of the superfluid density in the pnictide superconductor Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ across the superconducting dome

TL;DR

This study uses local measurement techniques to analyze the superfluid density in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$, revealing how it varies across different doping levels and indicating a complex interplay between magnetism and superconductivity.

Contribution

It provides the first local measurements of superfluid density across the superconducting dome in this material, highlighting doping-dependent changes and the nature of the superconducting gap.

Findings

Superfluid density sharply decreases on the underdoped side.

Fully gapped superconductivity at optimal doping.

Steep variation of superfluid density near $T_c$ in underdoped and optimally doped samples.

Abstract

We measure the penetration depth $\lambda_{ab}(T)$ in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ using local techniques that do not average over the sample. The superfluid density $\rho_s(T)\equiv1/\lambda_{ab}(T)^2$ has three main features. First, $\rho_s(T=0)$ falls sharply on the underdoped side of the dome. Second, $\lambda_{ab}(T)$ is flat at low $T$ at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, $\rho_s(T)$ varies steeply near $T_c$ for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.