Superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductor

TL;DR

This study investigates the superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$, revealing two anisotropic s-wave-like gaps with no nodes, supported by spectroscopy and specific heat measurements, challenging simple d-wave or single-gap models.

Contribution

The paper provides experimental evidence for two anisotropic s-wave-like gaps in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ using Andreev spectroscopy and specific heat analysis, clarifying the gap symmetry.

Findings

Two anisotropic superconducting gaps identified

Gaps show 25-30% anisotropy with no nodes

s-wave model explains temperature dependence

Abstract

We report on the Andreev spectroscopy and specific heat of high-quality single crystals BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps $\Delta_L \approx 3.2 \textendash 4.5$\,meV, $\Delta_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_xk_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps $\Delta_L \approx 3.2$\,meV and $\Delta_S \approx 1.6$\,meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers a further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.