Optical investigations of chemically pressurized EuFe2(As1-xPx)2: An s-wave superconductor with strong interband interactions

Superconducting EuFe2(As0.82P0.18)(2) single crystals are investigated by broad-band infrared spectroscopy. Below T-c = 28 K, a superconducting gap forms at 2 Delta(0) = 9.5 meV = 3.7k(B)T(c), causing the reflectivity to sharply rise to unity at low frequency. In the range of the gap, the optical conductivity can be perfectly described by BCS theory with an s-wave gap and no nodes. From our analysis of the temperature-dependent conductivity and spectral weight at T > T-c, we deduce an increased interband coupling between hole and electron sheets on the Fermi surface when T approaches T-c.