Doping evolution of the absolute value of the London penetration depth and superfluid density in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study measures the zero-temperature London penetration depth in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals across doping levels, revealing a significant increase in penetration depth in the coexistence region of antiferromagnetism and superconductivity, indicating evolving gap anisotropy.

Contribution

It provides the first systematic doping-dependent measurements of $ ext{λ}_{ab}(0)$ and superfluid density in this compound, highlighting the evolution of superconducting gap anisotropy.

Findings

$ ext{λ}_{ab}(0)$ increases sharply near the phase boundary.

Superfluid density shows signs of increasing gap anisotropy.

Coexistence of antiferromagnetism and superconductivity affects penetration depth.

Abstract

The zero temperature value of the in-plane London penetration depth, $\lambda_{ab}(0)$, has been measured in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ as a function of the Co concentration, $x$, across both the underdoped and overdoped superconducting regions of the phase diagram. For $x\gtrsim0.047$, $\lambda_{ab}(0)$ has been found to have values between 120 $\pm$ 50~nm and 300 $\pm$ 50~nm. A pronounced increase in $\lambda_{ab}(0)$, to a value as high as 950 $\pm$ 50~nm, has been observed for $x\lesssim0.047$, corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent $\lambda_{ab}(0)$ has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.