Electron-doping dependence of the anisotropic superconductivity in BaFe$_{2-x}$Ni$_{x}$As$_2$

TL;DR

This study investigates how electron doping affects the anisotropic superconductivity in BaFe$_{2-x}$Ni$_{x}$As$_2$, revealing doping-dependent changes in upper critical fields and anisotropy linked to Fermi surface topology.

Contribution

It provides a comprehensive analysis of the doping dependence of upper critical fields and anisotropy in BaFe$_{2-x}$Ni$_{x}$As$_2$ using magnetotransport measurements across the superconducting dome.

Findings

Anisotropy $b3(T)$ increases with temperature for all dopings.

Zero-temperature anisotropy $b3(0)$ peaks in the overdoped regime.

Superconductivity anisotropy is influenced by Fermi surface topology and impurity scattering.

Abstract

The upper critical field ($H_{c2}$) in superconducting BaFe$_{2-x}$Ni$_{x}$As$_2$ single crystals has been determined by magnetotransport measurements down to 0.6 K over the whole superconducting dome with $0.065 \leqslant x \leqslant 0.22$, both for the inter-plane ($H \parallel c$, $H_{c2}^{c}$) and in-plane ($H \parallel ab$, $H_{c2}^{ab}$) field directions in static magnetic fields up to 16 T and pulsed magnetic fields up to 60 T. The temperature dependence of $H_{c2}^{ab}$ follows the Werthamer-Helfand-Hohenberg (WHH) model incorporating orbital and spin paramagnetic effects, while $H_{c2}^{c}(T)$ can only be described by the effective two-band model with unbalanced diffusivity. The anisotropy of the upper critical fields, $\gamma (T)=H_{c2}^{ab}/H_{c2}^{c}$ monotonically increases with increasing temperature for all dopings, and its zero-temperature limit, $\gamma (0)$, has an asymmetric doping dependence with a significant enhancement in the overdoped regime, where the optimally doped compound has the most isotropic superconductivity. Our results suggest that the anisotropy in the superconductivity of iron pnictides is determined by the topology of the Fermi surfaces together with the doping-induced impurity scattering.