Not all iron pnictide superconductors are the same

I am not working on the new iron pnictide superconductors and so am only casually following the field. Here is one recent paper that got my attention.

Nodal versus Nodeless Behaviors of the Order Parameters of LiFeP and LiFeAs Superconductors from Magnetic Penetration-Depth Measurements
K. Hashimoto, S. Kasahara, R. Katsumata, Y. Mizukami, M. Yamashita, H. Ikeda, T. Terashima, A. Carrington, Y. Matsuda, and T. Shibauchi

The temperature dependence of the London penetration depth [which determines the superfluid density] is strikingly different for LiFeP and LiFeAs. This suggests that for the former there are nodes in the superconducting gap.

Furthermore, they correlate the presence of nodes in the superconducting energy gap with  the height of the pnictogen atoms (Pn=As or P) above the plane of the Fe atoms in the crystal structure.