Microwave Penetration Depth and Quasiparticle Conductivity in   PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap Superconductor
  In-plane microwave penetration depth $\lambda_{ab}$ and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO$_{1-y}$ ($T_c\approx 35$ K) by using a sensitive superconducting cavity resonator. $\lambda_{ab}(T)$ shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap $\Delta({\bf k})$. The temperature dependence of the superfluid density demonstrates that the gap is non-zero ($\Delta/k_BT_c\gtrsim 1.6$) all over the Fermi surface. The microwave conductivity below $T_c$ exhibits an enhancement larger than the coherence peak, reminiscent of high-$T_c$ cuprate superconductors. 
K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada, R. Okazaki, H.   Shishido, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamot...