Enhancement of the superconducting gap by nesting in CaKFe4As4 - a new high temperature superconductor

TL;DR

This study investigates CaKFe4As4, a high-temperature superconductor with Tc of 35K, revealing that its superconducting gap is strongly influenced by Fermi surface nesting, supporting a multiband pairing mechanism.

Contribution

The paper combines experimental ARPES data and DFT calculations to demonstrate the role of Fermi surface nesting in the superconducting gap structure of CaKFe4As4, a stoichiometric high-Tc superconductor.

Findings

Superconducting gap is nearly isotropic but varies across Fermi surface sheets.

Maximum gap occurs on Fermi surface sheets with the best nesting conditions.

Results support a multiband superconductivity model with interband interactions.

Abstract

We use high resolution angle resolved photoemission spectroscopy and density functional theory with experimentally obtained crystal structure parameters to study the electronic properties of CaKFe4As4. In contrast to related CaFe2As2 compounds, CaKFe4As4 has high Tc of 35K at stochiometric composition. This presents unique opportunity to study properties of high temperature superconductivity of iron arsenic superconductors in absence of doping or substitution. The Fermi surface consists of three hole pockets at $\Gamma$ and two electron pockets at the $M$ point. We find that the values of the superconducting gap are nearly isotropic, but significantly different for each of the FS sheets. Most importantly we find that the overall momentum dependence of the gap magnitudes plotted across the entire Brillouin zone displays a strong deviation from the simple cos(kx)cos(ky) functional form of the gap function, proposed in the scenario of the Cooper-pairing driven by a short range antiferromagnetic exchange interaction. Instead, the maximum value of the gap is observed for FS sheets that are closest to the ideal nesting condition in contrast to the previous observations in some other ferropnictides. These results provide strong support for the multiband character of superconductivity in CaKFe4As4, in which Cooper pairing forms on the electron and the hole bands interacting via dominant interband repulsive interaction, enhanced by FS nesting}.