Optical properties of the iron-based superconductor LiFeAs single crystal

We have measured the reflectivity spectra of the LiFeAs (T_c = 17.6 K) single crystal in the temperature range from 4 to 300 K. In the superconducting (SC) state (T < T_c), the clean opening of the optical absorption gap was observed below 25 cm^-1, indicating an isotropic full gap formation. In the normal state (T > T_c), the optical conductivity spectra display typical metallic behavior with the Drude-type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this compound. A theoretical analysis of the low-temperature data (T = 4 K < T_c) also suggests that two SC gaps best fit the data and their values were estimated as Δ₁ = 3.3 meV and Δ₂ = 1.59 meV, respectively. Using the Ferrell-Glover-Tinkham sum rule and dielectric function ₁ (ω), the plasma frequency of the SC condensate (ω_ps) is consistently estimated to be 6665 cm^-1, implying that about 65% of the free carriers of the normal state condense into the SC condensate. To investigate the various interband transition processes (for ω > 200 cm^-1), we have also performed the local-density approximation band calculation and calculated the optical spectra of the interband transitions. This theoretical result provided qualitative agreement with the experimental data below 4000 cm^-1 .