Point-contact spectroscopy in Co-doped CaFe2As2: nodal superconductivity and topological Fermi surface transition

TL;DR

This study uses point-contact spectroscopy to reveal nodal superconductivity and a topological Fermi surface transition in Co-doped CaFe2As2, highlighting distinct gap structures on different Fermi surface sheets.

Contribution

It provides experimental evidence of nodal gaps and Fermi surface topology changes in Ca(Fe_{1-x}Co_x)2As2, advancing understanding of superconductivity in 122 iron pnictides.

Findings

Presence of a large isotropic gap on quasi-2D electron Fermi surfaces

Smaller anisotropic (possibly nodal) gap on 3D hole Fermi surfaces

Consistency with theoretical models predicting nodal superconductivity

Abstract

We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds.