Anisotropy of the iron pnictide superconductorBa(Fe1−xCox)2As2(x=0.074, Tc=23 K)

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 $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ ($x=0.074$ and ${T}_{c}\ensuremath{\sim}23\text{ }\text{K}$). 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 ${\ensuremath{\gamma}}_{\ensuremath{\rho}}={\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{a}$ is about $4\ifmmode\pm\else\textpm\fi{}1$ just above ${T}_{c}$ and becomes half of that at room temperature. The anisotropy of the upper critical field, ${\ensuremath{\gamma}}_{H}={H}_{c2,ab}/{H}_{c2,c}$, as determined from specific-heat measurements close to ${T}_{c}$ is in the range of 2.1--2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}={\ensuremath{\lambda}}_{c}/{\ensuremath{\lambda}}_{ab}$, was recorded from tunnel diode resonator measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, ${\ensuremath{\gamma}}_{j}={j}_{c,ab}/{j}_{c,c}$, as determined from both direct transport measurements $(\ensuremath{\sim}1.5)$ and from the analysis of the magnetization data $(\ensuremath{\sim}3)$. Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compounds and bear little resemblance to cuprates.