Multiband effects on the upper critical field angular dependence of 122-family iron pnictide superconductors

Abstract

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(Fe 1−x Co x ) 2 As 2 (x = 0.065) single crystal, in magnetic fields up to 9 T and with different orientations θ relative to the crystal c axis. A significant ρ(T ) H,θ rounding is observed just above the superconducting critical temperature T c due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, H c2 (θ ). In spite of the relatively small anisotropy of this compound, it is found that H c2 (θ ) presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large θ (typically above ∼ 60 o ). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar H c2 (θ ) anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near T c , a result that differs from the ones obtained by using a single-band model. c2 18-21 However, an anomalous behavior has been reported in some of these compounds, 22,23 qualitatively similar to the behavior observed in other two-band superconductors, such as MgB 2 . 24,25