Temperature dependence of the superconducting energy gaps in Ca_9.35La_0.65(Pt₃As₈)(Fe₂As₂)₅ single crystal

We measured the optical reflectivity R(ω) for an underdoped (Ca_0.935La_0.065)₁₀(Pt₃As₈)(Fe₂As₂)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 Δ₁ = 30 and Δ₂ = 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.