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

TL;DR

This study investigates the angular dependence of the upper critical field in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ superconductors, revealing multiband effects that cause deviations from single-band models, using advanced fluctuation analysis techniques.

Contribution

The paper introduces a generalized analysis method for $H_{c2}( heta)$, demonstrating multiband effects in iron pnictide superconductors beyond traditional single-band models.

Findings

$H_{c2}( heta)$ deviates from single-band Ginzburg-Landau predictions at large angles.

Multiband effects cause significant anisotropy deviations, especially above 60 degrees.

Effective anisotropy remains nearly temperature independent near $T_c$.

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 $\theta$ 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 $\sim60^o$). 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.