Optical conductivity as a probe of a hidden Fermi-liquidbehavior in BaFe_1.904Ni_0.096As₂

We measured the infrared reflectivity of BaFe_1.904Ni_0.096As₂ single crystal from room temperature down to 20 K. Two Drude terms and a Lorentz term well describe the real part of the optical conductivity σ₁(ω). We fit the reciprocal of static optical conductivity 1/σ₁(0) by the power law ρ(T)=ρ₀+ATⁿ with n=1.6. The “broad” Drude component exhibits an incoherent background with a T-independent scattering rate 1/τ_b, while the other “narrow” one reveals a T-quadratic scattering rate 1/τ_n, indicating a hidden Fermi-liquid behavior in BaFe_1.904Ni_0.096As₂ compound.