Nodeless vs nodal order parameters in LiFeAs and LiFeP superconductors

TL;DR

This study compares the low-energy quasiparticle excitations in LiFeAs and LiFeP superconductors, revealing fully gapped and nodal states respectively, and links the nodal behavior to structural parameters across iron-pnictide families.

Contribution

It provides the first systematic comparison of quasiparticle excitations in LiFeAs and LiFeP, establishing a correlation between nodal superconductivity and pnictogen height.

Findings

LiFeAs shows flat low-temperature penetration depth, indicating a fully gapped state.

LiFeP exhibits linear temperature dependence, indicating a nodal superconducting gap.

Nodal superconductivity correlates with pnictogen height below ~1.33 Å.

Abstract

High-precision measurements of magnetic penetration depth $\lambda$ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting low-energy quasiparticle excitations. In LiFeAs the low-temperature $\lambda(T)$ shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a $T$-linear dependence of superfluid density $\propto \lambda^{-2}$, indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of $\sim 1.33$\,\AA.