The London penetration depth in single crystals of Ba(Fe_{1-x}Co_x)_2As_2 at various doping levels

TL;DR

This study measures the London penetration depth in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals across various doping levels, revealing a consistent power-law behavior and insights into superconducting carrier density.

Contribution

It provides detailed measurements of $ ext{λ}(T)$ across multiple doping levels, highlighting the robustness of the power-law behavior in the penetration depth.

Findings

Carrier density decreases sharply in underdoped samples.

The penetration depth follows a power law with exponent between 2 and 2.5.

Robust power-law behavior persists across all doping levels.

Abstract

The London penetration depth $\lambda(T)$ has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ using the tunnel diode resonator technique. The measured doping levels of $x=$~0.038, 0.047, 0.058, 0.074 and 0.10 range from underdoped to overdoped concentrations. The measurements have shown that the density of carriers participating in superconductivity decreases sharply in the underdoped regime, but the penetration depth as a function of temperature exhibits a robust power law, $\Delta\lambda(T)\sim T^{n}$, for all measured dopings, with $n$ between 2 and 2.5. We discuss the implications of these results and possible interpretations of such robust behavior.