Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

TL;DR

This study reveals complex anisotropic superconducting gaps in a slightly overdoped iron pnictide superconductor, challenging existing theories and offering new insights into its superconducting mechanisms.

Contribution

First detailed 3D ARPES mapping of SC-gap anisotropy in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, revealing unexpected gap structures.

Findings

Inner electron Fermi surface has isotropic gap

Outer electron Fermi surface shows anisotropic gap with nodes or minima

Results challenge existing theoretical models of superconductivity

Abstract

We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.1$), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors.