Large increase of the anisotropy factor in the overdoped region of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ as probed by fluctuation spectroscopy

TL;DR

This study investigates how the superconducting anisotropy factor in Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ increases significantly with doping, using fluctuation spectroscopy and a 3D-anisotropic Ginzburg-Landau model.

Contribution

It provides quantitative confirmation of the doping-dependent increase of the anisotropy factor in an iron-based superconductor using fluctuation spectroscopy.

Findings

Confirmed applicability of 3D-anisotropic Ginzburg-Landau approach

Quantified doping dependence of coherence length and anisotropy factor

Demonstrated large increase of anisotropy factor with doping

Abstract

We study the diamagnetism induced by thermal fluctuations above the superconducting transition of the iron pnictide Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ with different doping levels. The measurements are performed with magnetic fields up to 7 T applied in the two main crystal directions. These data provide double information: first, they confirm at a quantitative level the applicability to these materials of a 3D-anisotropic Ginzburg-Landau approach valid in the finite field regime. Then, they allow to determine the doping-level dependence of the in-plane coherence length and of the superconducting anisotropy factor, $\gamma$. Our results provide a stringent confirmation of the large increase of $\gamma$ with the doping level, as recently proposed from magnetoresistivity measurements. The implications of the applicability of the model used to a multiband superconductor are discussed.