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

Abstract

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 λ ab in single crystals of electron-doped PrFeAsO 1 - y ( y ∼ 0.1 ) and hole-doped Ba 1 - x K x Fe 2 As 2 ( x ≈ 0.55 ) are presented. In clean crystals of Ba 1 - x K x Fe 2 As 2 , as well as in PrFeAsO 1 - 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 λ ab 2 ( 0 ) / λ ab 2 ( T ) in both systems is most consistent with the existence of two different gaps. In Ba 1 - x K x Fe 2 As 2 , 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 c 1 in PrFeAsO 1 - y by using an array of micro-Hall probes. The temperature dependence of H c 1 saturates at low temperatures, fully consistent with the superfluid density determined by microwave measurements. The anisotropy of H c 1 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.