Unconventional superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As observed by angle-resolved photoemission spectroscopy

TL;DR

This study uses high-resolution ARPES to reveal an unconventional, strongly coupled superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As, challenging traditional phonon-mediated pairing theories.

Contribution

It provides direct experimental evidence of an unconventional superconducting gap structure and strong coupling in NaFe$_{0.95}$Co$_{0.05}$As, using angle-resolved photoemission spectroscopy.

Findings

Nearly isotropic superconducting gaps observed on all Fermi surfaces.

Large gap ratios indicating strong coupling ($2\Delta/k_{B}T_{c}\\sim8$).

Superconducting gap remains unchanged across $T_c$ while coherence vanishes.

Abstract

We have performed high resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe$_{0.95}$Co$_{0.05}$As ($T_{c}\sim$18 K). We observed a hole-like Fermi surface around the zone center and two electron-like Fermi surfaces around the M point which can be connected by the $Q=(\pi, \pi)$ wavevector, suggesting that scattering over the near-nested Fermi surfaces is important to the superconductivity of this "111" pnicitide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below $T_c$ on all three Fermi surfaces. Upon increasing temperature through $T_c$, the gap size shows little change while the coherence vanishes. Large ratios of $2\Delta/k_{B}T_{c}\sim8$ are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.