Temperature dependence of the superconducting energy gaps in Ca(9.35)La(0.65)(Pt(3)As(8))(Fe(2)As(2))(5) single crystal

We measured the optical reflectivity R(ω) for an underdoped (Ca(0.935)La(0.065))(10)(Pt(3)As(8))(Fe(2)As(2))(5) single crystal and obtained the optical conductivity [Formula: see text] using the K-K transformation. The normal state [Formula: see text] at 30 K is well fitted by a Drude-Lorentz model with two Drude components (ω(p1) = 1446 cm(−1) and ω(p2) = 6322 cm(−1)) and seven Lorentz components. Relative reflectometry was used to accurately determine the temperature dependence of the superconducting gap at various temperatures below T(c). The results clearly show the opening of a superconducting gap with a weaker second gap structure; the magnitudes for the gaps are estimated from the generalized Mattis-Bardeen model to be Δ(1) = 30 and Δ(2) = 50 cm(−1), respectively, at T = 8 K, which both decrease with increasing temperature. The temperature dependence of the gaps was not consistent with one-band BCS theory but was well described by a two-band (hence, two gap) BCS model with interband interactions. The temperature dependence of the superfluid density is flat at low temperatures, indicating an s-wave full-gap superconducting state.