Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

In this article, we review our recent studies of microwave penetration depth, lower critical fields, and quasiparticle conductivity in the superconducting state of Fe-arsenide superconductors. High-sensitivity microwave surface impedance measurements of the in-plane penetration depth lambda(ab) in single crystals of electron-doped PrFeAsO1-y(y similar to 0.1) and hole-doped Ba1-xKxFe2As2(x approximate to 0.55) are presented. In clean crystals of Ba1-xKxFe2As2, as well as in PrFeAsO1-y crystals, the penetration depth shows flat temperature dependence at low temperatures, indicating that the superconducting gap opens all over the Fermi surface. The temperature dependence of superfluid density lambda(2)(ab)(0)/lambda(2)(ab)(T) in both systems is most consistent with the existence of two different gaps. In Ba1-xKxFe2As2, we find that the superfluid density is sensitive to degrees of disorder inherent in the crystals, implying unconventional impurity effect. We also determine the lower critical field H-c1 in PrFeAsO1-y by using an array of micro-Hall probes. The temperature dependence of H-c1 saturates at low temperatures, fully consistent with the superfluid density determined by microwave measurements. The anisotropy of H-c1 has a weak temperature dependence with smaller values than the anisotropy of upper critical fields at low temperatures, which further supports the multi-gap superconductivity in Fe-arsenide systems. The quasiparticle conductivity shows an enhancement in the superconducting state, which suggests the reduction of quasiparticle scattering rate due to the gap formation below T-c. From these results, we discuss the structure of the superconducting gap in these Fe-arsenides. in comparison with the high-T-c cuprate superconductors. (C) 2009 Elsevier B.V. All rights reserved.