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

I F Llovo,S Salem-Sugui,C Carballeira,A Pereiro,D Sóñora,A S Sefat,J J Ponte,J Mosqueira

doi:10.1038/s41598-021-90858-4

I F Llovo, S Salem-Sugui + Show 6 more

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https://doi.org/10.1038/s41598-021-90858-4

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Abstract

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(hbox {Fe}_{1-x}hbox {Co}_{{x}})_2hbox {As}_2 (x=0.065) single crystal, in magnetic fields up to 9 T and with different orientations theta relative to the crystal c axis. A significant rho (T)_{H,theta } 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}(theta ). In spite of the relatively small anisotropy of this compound, it is found that H_{c2}(theta ) presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large theta (typically above sim 60^o). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar H_{c2}(theta ) 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.

Highlights

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(Fe1−x Cox)2As2 ( x = 0.065 ) single crystal, in magnetic fields up to 9 T and with different orientations θ relative to the crystal c axis
The angular dependence of the upper critical magnetic field, Hc2(θ ), where θ is the angle between the applied magnetic field and the crystal c axis, has been less studied
We present measurements of the in-plane resistivity versus temperature under magnetic fields with different amplitudes and orientations with respect to the crystal c axis, in an optimally-doped Ba(Fe1−xCox )2As2 (OP-BaFeCoAs) single crystal

Summary

Introduction

In spite of the relatively small anisotropy of this compound, it is found that Hc2(θ ) presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, for large θ (typically above ∼ 60o ) These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar Hc2(θ ) anomalies. There is a fundamental interest in discovering the pairing mechanism responsible for their high critical temperature, which could be related to the one of cuprates They present unconventional superconducting properties associated to their multiband electronic structure, with energy gaps that depend on the doping level and on the pressure (external or chemical)[4,5,6,7,8]. Has been reported in some of these compounds[22,23], qualitatively similar to the behavior observed in other twoband superconductors, such as MgB224,25

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