Probing the superconducting energy gap from infrared spectroscopy on a Ba0.6K0.4Fe2As2 single crystal with Tc=37 K

We performed optical spectroscopy measurement on a superconducting Ba0.6K0.4Fe2As2 single crystal with T-c = 37 K. Formation of the superconducting energy gaps in the far-infrared reflectance spectra below T-c is clearly observed. A flat and close to unity reflectance is observed roughly below 150 cm(-1) for T < T-c, following an s-wave pairing line shape. A more rapid decrease occurs near 200 cm(-1), leading to a peak in the ratio of the reflectance at T << T-c over that for T >= T-c. We determined the absolute value of the penetration depth at 10 K as lambda similar or equal to 2000 +/- 80 angstrom. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at low energy scale, less than 6 Delta.