Nodal versus nodeless superconductivity in iso-electronic LiFeP and LiFeAs

TL;DR

This study explains why LiFeP exhibits nodal superconductivity while LiFeAs is nodeless by analyzing their electronic structures and pairing interactions, revealing the orbital-specific competition that determines the superconducting gap symmetry.

Contribution

The paper introduces a detailed Eliashberg theory analysis showing how orbital-dependent pairing interactions lead to nodal versus nodeless superconductivity in closely related compounds.

Findings

LiFeP has a dominant $d_{xy}$ orbital pairing symmetry with nodes.

LiFeAs exhibits a uniform pairing symmetry across orbitals, resulting in nodeless gaps.

LiFeP's superconducting transition temperature is lower due to weak pairing of $d_{xz/yz}$ electrons.

Abstract

Nodal superconductivity is observed in LiFeP while its counterpart LiFeAs with similar topology and orbital content of the Fermi surfaces is a nodeless superconductor. We explain this difference by solving, in the two-Fe Brillouin zone, the frequency-dependent Eliashberg equations with spin-fluctuation mediated pairing interaction. Because of Fermi surface topology details, in LiFeAs all the Fe-$t_{2g}$ orbitals favor a common pairing symmetry. By contrast, in LiFeP the $d_{xy}$ orbital favors a pairing symmetry different from $d_{xz/yz}$ and their competition determines the pairing symmetry and the strength of the superconducting instability: $d_{xy}$ orbital strongly overcomes the others and imposes the symmetry of the superconducting order parameter. The leading pairing channel is a $d_{xy}$-type state with nodes on both hole and electron Fermi surfaces. As a consequence, the $d_{xz/yz}$ electrons weakly pair leading to a reduced transition temperature in LiFeP.