Electronic, magnetic and optical properties of two Fe-based superconductors and related parent compounds

We have investigated the electronic, magnetic, and optical properties of two Fe-based superconductors and related parent compounds via three powerful techniques: scanning tunneling microscopy/spectroscopy (STM/S), high-temperature vibrating sample magnetometry, and optical transmission spectroscopy (OTS). Below the superconducting transition temperature T(c) similar to 48 K, the STM/S of polycrystalline NdFeAsO(0.86)F(0.14) reveals a single-gap feature. The quantitative fitting of STS data results in Bardeen-Cooper-Schrieffer-like temperature dependence of the energy gap Delta(T), with 2 Delta(0)/k(B)T(c) similar to 4.3. The tunneling spectra of BaFe(2)As(2) single crystals show no evidence for the opening of a gap below the magnetic/structural transition temperature T(MS) similar to 140 K. This transition also had little impact on the transmission spectra in the wavelength range between 400 and 2400 nm. But its effect on the magnetic properties is dramatic, as reflected in the unusual magnetic susceptibility over a wide temperature range.