FeTe₀.₅₅Se₀.₄₅: a multiband superconductor in the clean and dirty limit

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The detailed optical properties of the multiband iron-chalcogenide superconductor FeTe₀.₅₅Se₀.₄₅ have been reexamined for a large number of temperatures above and below the critical temperature Tc=14 K for light polarized in the a-b planes. Instead of the simple Drude model that assumes a single band, above Tc the normal-state optical properties are best described by the two-Drude model that considers two separate electronic subsystems; we observe a weak response (ωₚ,D;₁≃ 3000 cm⁻¹) where the scattering rate has a strong temperature dependence (1/τD,₁≃ 32 cm⁻¹ for T Tc), and a strong response (ωₚ,D;₂≃ 14500 cm⁻¹) with a large scattering rate (1/τD,₂≃ 1720 cm⁻¹) that is essentially temperature independent. The multiband nature of this material precludes the use of the popular generalized-Drude approach commonly applied to single-band materials, implying that any structure observed in the frequency dependent scattering rate 1/τ(ω) is spurious and it cannot be used as the foundation for optical inversion techniques to determine an electron-boson spectral function α² F(ω). Below Tc the optical conductivity is best described using two superconducting optical gaps of 2Δ₁≃ 45 and 2Δ₂ ≃ 90 cm⁻¹ applied to the strong and weak responses, respectively. The scattering rates for these two bands are vastly different at low temperature, placing this material simultaneously in both clean and dirty limit. Interestingly, this material falls on the universal scaling line initially observed for the cuprate superconductors.