Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFe$_2$As$_2$ under pressure

TL;DR

This study combines experimental spectroscopy and theoretical modeling to reveal a topological Fermi-surface transition and the emergence of horizontal nodes in the superconducting order parameter of CaFe$_2$As$_2$ under pressure.

Contribution

It provides the first detailed experimental and theoretical analysis linking Fermi-surface topology changes to superconducting gap symmetry in CaFe$_2$As$_2$ under pressure.

Findings

Fermi-surface undergoes a topological transition under pressure.

Superconducting order parameter develops horizontal nodes.

Experimental results agree with 3D Andreev reflection model and ab-initio calculations.

Abstract

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe$_2$As$_2$, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe$_2$As$_2$ crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the hole-like Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.