Nodal versus Nodeless Behaviors of the Order Parameters of LiFeP and LiFeAs Superconductors from Magnetic Penetration-Depth Measurements

摘要

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