Optical properties of optimally doped single-crystal Ca8.5La1.5(Pt3As8)(Fe2As2)5

We have measured the reflectivity of the optimally doped Ca8.5La1.5(Pt3As8)(Fe10As10) single crystal (Tc=32.8K) over the broad frequency range from 40cm-1 to 12 000cm-1 and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region (<1000cm-1) in the normal state (80 K <T≤300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around 100cm-1 and continuously evolves into the fully opened superconducting (SC) gap structure below Tc. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above Tc into the SC gap structure below Tc. The extracted two SC gap sizes are ΔS=4.9 meV and ΔL=14.2 meV, suggesting Ca8.5La1.5(Pt3As8)(Fe10As10) as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.