Anisotropy of the iron pnictide superconductor Ba(Fe_1−xCo_x)₂As₂ (x=0.074, T_c=23 K)

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−xCo_x)₂As₂ (x=0.074 and T_c∼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 γ_ρ=ρ_c/ρ_a is about 4±1 just above T_c and becomes half of that at room temperature. The anisotropy of the upper critical field, γ_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, γ_λ=λ_c/λ_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, γ_j=j_c,ab/j_c,c, as determined from both direct transport measurements (∼1.5) and from the analysis of the magnetization data (∼3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compounds and bear little resemblance to cuprates.