Millimeter-wave study of London penetration depth temperature dependence in Ba(Fe(0.926)Co(0.074))(2)As(2) single crystal

In-plane surface Ka-band microwave impedance of optimally doped single crystals of the Fe-based superconductor Ba(Fe0.926Co0.074)(2)As-2 (T-c = 22.8 K) was measured. Sensitive sapphire disk quasi-optical resonator with high-T-c cuprate conducting endplates was developed specially for Fe-pnictide superconductors. It allowed finding temperature variation of London penetration depth in a form of power law, namely Delta lambda(T) similar to T-n with n = 2.8 from low temperatures up to at least 0.6T(c) consisted with radio-frequency measurements. This exponent points towards nodeless state with pairbreaking scattering, which can support one of the extended s-pairing symmetries. The dependence lambda(T) at low temperatures is well described by one superconducting small-gap (Delta congruent to 0.75 in kT(c) units, where k is Boltzmann coefficient) exponential dependence. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3660321