Terahertz conductivity in FeSe_0.5Te_0.5 superconducting films

Complex conductivity of FeSe0.5Te0.5 superconducting films has been investigated using backward-wave-oscillator spectroscopy in the terahertz frequency range. The normal state conductivity corresponds well to the low-frequency limit of a Drude metal. In the superconducting state, the real part of the complex conductivity is not completely suppressed, indicating the existence of a strongly anisotropic gap. Weak features in the conductivity are consistent with the superconducting gap value close to 12 cm−1(1.5 meV). The experimental results are discussed within the framework of a two-gap model based on the s± symmetry of the order parameter.