Detailed measurements of the global and local electromagnetic properties of Ba(Fe0.93Co0.07)2As2 single crystals are reported. Analysis of the irreversible magnetic response provides strong evidence for similar vortex physics in this Fe-based pnictide superconductor to the high-Tc cuprates, such as Y-Ba-Cu-O or Nd-Ce-Cu-O. In particular, we have found a nonmonotonic "fishtail" magnetization in M (H, T = const) loops and its signature is also present in M (H = const, T ) scans. The supercurrent density is evaluated by using several techniques, including direct transport measurements. At 5 K we estimate its value to be a moderate j ≈ 2.6 ± 0.2 × 10 5 A/cm 2 . Analysis of the magnetic relaxation is consistent with the collective pinning and creep models (weak pinning and fast creep) and suggests a crossover from the collective to the plastic creep regime in fields exceeding the value corresponding to the maximum in fishtail magnetization.

The thermal conductivity κ of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c axis. A residual linear term at T → 0, κ0/T , is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of κ0/T in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity ρ0 was ten times larger. This independence of κ0/T on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate Γc for suppressing Tc to zero; the magnitude of κ0/T , and its dependence on current direction and on magnetic field; the temperature dependence of κ(T ).

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