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 in single crystals of electron-doped PrFeAsO1-y (y~0.1) and hole-doped Ba1-xKxFe2As2 (x~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 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.

12 pages, 11 figures. Review article in Physica C Special Issue on Superconductivity of Iron Pnictides

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Cited by in corpus (10)

• Superconductivity in Iron Compounds
• London Penetration Depth in Iron - based Superconductors
• Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
• Temperature dependence of lower critical field Hc1(T) shows nodeless superconductivity in FeSe
• Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity
• Using electron irradiation to probe iron - based superconductors
• Small anisotropy of the lower critical field and $s_{\pm }$-wave two-gap feature in single crystal LiFeAs
• Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO
• Multiband strong-coupling superconductors with spontaneously broken time-reversal symmetry
• Millimeter-wave study of London penetration depth temperature dependence in Ba(Fe0.926Co0.074)2As2 single crystal