Are there nodes in LaFePO, BaFe$_2$(AsP)$_2$, and KFe$_2$As$_2$ ?

TL;DR

This paper critically reexamines experimental evidence for superconducting gap nodes in three Fe-pnictide compounds, revealing inconsistencies in data regarding their cleanliness and challenging previous claims of nodal gaps.

Contribution

It provides a comprehensive analysis showing that some compounds previously thought to have nodal gaps are actually dirty, questioning the existence of nodes in these materials.

Findings

LaFePO and BaFe2(AsP)2 are extremely dirty superconductors.

KFe2As2 can be clean, unlike the other two compounds.

Conflicting experimental data cast doubt on the nodal gap hypothesis.

Abstract

We reexamined the experimental evidences for the possible existence of the superconducting (SC) gap nodes in the three most suspected Fe-pnictide SC compounds: LaFePO, BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$, and KFe$_2$As$_2$. We showed that while the $T$-linear temperature dependence of the penetration depth $\lambda(T)$ of these three compounds indicate extremely clean nodal gap superconductors, the thermal conductivity data $\lim_{T,H \rightarrow 0} \kappa_S (H,T)/T$ unambiguously showed that LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ are extremely dirty, while KFe$_2$As$_2$ can be clean. This apparently conflicting experimental data casts a serious doubt on the nodal gap possibility on LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$.