Local measurement of the penetration depth in the pnictide superconductor Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$

TL;DR

This study employs magnetic force microscopy and SQUID susceptometry to locally measure the penetration depth and superfluid density in a pnictide superconductor, revealing a slower temperature dependence and spatial uniformity despite disorder.

Contribution

It demonstrates that MFM can accurately measure the absolute penetration depth and its temperature dependence in a pnictide superconductor, providing insights into its superfluid density behavior.

Findings

Penetration depth varies slowly with temperature, fitting a clean two-band model.

Superfluid density is spatially uniform despite vortex pinning disorder.

MFM effectively measures absolute penetration depth and temperature dependence.

Abstract

We use magnetic force microscopy (MFM) and scanning SQUID susceptometry to measure the local superfluid density $\rho_{s}$ in Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ single crystals from 0.4 K to the critical temperature $T_c=18.5$ K. We observe that the penetration depth $\lambda$ varies about ten times more slowly with temperature than previously published, with a dependence that can be well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $\lambda$, as well as obtain its temperature dependence and spatial homogeneity. We find $\rho_{s}$ to be uniform despite the highly disordered vortex pinning.