Nodeless vs nodal order parameters in LiFeAs and LiFeP superconductors
  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. 
K. Hashimoto, S. Kasahara, R. Katsumata, Y. Mizukami, M. Yamashita, H.   Ikeda, T. Terashima, A. Carrington, Y. Matsuda, and...
cond-mat.supr-con