Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals

TL;DR

This study measures the London penetration depth in high-quality Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ crystals, revealing unconventional behavior suggestive of point nodes in the superconducting gap, contrasting with other iron-based superconductors.

Contribution

It provides evidence for a possible anisotropic s-wave gap with point nodes in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, highlighting a different gap structure from similar compounds.

Findings

Power-law temperature dependence of $\lambda(T)$ with n=2.4

Normal quasiparticles present down to 0.02$T_c$

Possible point nodes in the superconducting gap

Abstract

The London penetration depth, $\lambda(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $\lambda(T)$ follows a power-law, $\Delta \lambda (T) \sim T^n$ with $n=2.4 \pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $\lambda(T)$.