Nanoscale superconducting gap variations, strong coupling signatures and lack of phase separation in optimally doped BaFe1.86Co0.14As2

TL;DR

This study investigates nanoscale variations in the superconducting gap of optimally doped BaFe1.86Co0.14As2, revealing strong coupling signatures and no phase separation, with implications for understanding the pairing mechanism.

Contribution

It provides detailed tunneling data showing nanoscale gap variations and strong coupling features in an iron-based superconductor, highlighting the absence of phase separation.

Findings

Nanoscale gap variations correlate with Co dopant separation

Superconducting gap exceeds BCS weak coupling value

Spectral features suggest coupling to a bosonic mode

Abstract

We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Delta. The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance, match well with the average separation between the Co dopant ions in the superconducting FeAs planes. The tunneling spectra themselves possess a peak-dip-hump lineshape, suggestive of a coupling of the superconducting electronic system to a well-defined bosonic mode of energy 4.7 kBTC, such as the spin resonance observed recently in inelastic neutron scattering.