Microwave Penetration Depth and Quasiparticle Conductivity of PrFeAsO1-y Single Crystals: Evidence for a Full-Gap Superconductor

In-plane microwave penetration depth λ_ab and quasiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO_1-y (T_c≈35K) by using a sensitive superconducting cavity resonator. λ_ab(T) shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap Δ(k). The temperature dependence of the superfluid density demonstrates that the gap is nonzero (Δ/k_BT_c≳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.