Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2

TL;DR

This study measures various anisotropies in the optimally doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ superconductor, revealing low anisotropy levels indicative of three-dimensional electronic behavior.

Contribution

It provides comprehensive experimental measurements of anisotropies in resistivity, critical fields, penetration depth, and critical currents in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, highlighting its three-dimensional characteristics.

Findings

Resistivity anisotropy ratio ~4 near $T_c$

Upper critical field anisotropy ~2.1 to 2.6

London penetration depth anisotropy persists deep into superconducting state

Abstract

Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $\gamma_{\rho} = \rho_c /\rho_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $\gamma_{H} = H_{c2,ab} /H_{c2,c} $, as determined from specific heat measurements close to $T_c$, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, $\gamma_{\lambda}=\lambda_{c}/\lambda_{ab}$, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, $\gamma_j=j_{c,ab}/j_{c,c}$, as determined from both direct transport measurements ($\sim$1.5) and from the analysis of the magnetization data ($\sim$3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates.