High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting low-energy quasiparticle excitations. In LiFeAs the low-temperature λ(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 ∝λ−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 ∼1.33\,\AA.
@article{arxiv.1107.4505,
title = {Nodeless vs nodal order parameters in LiFeAs and LiFeP superconductors},
author = {K. Hashimoto and S. Kasahara and R. Katsumata and Y. Mizukami and M. Yamashita and H. Ikeda and T. Terashima and A. Carrington and Y. Matsuda and T. Shibauchi},
journal= {arXiv preprint arXiv:1107.4505},
year = {2012}
}